Fix pelican warnings

[moorepants]

Noticed these on the travis builds:

WARNING: Defining PLUGIN_PATHS setting as string has been deprecated (should be a list)

WARNING: %s usage in CATEGORY_FEED_ATOM is deprecated, use {slug} instead.

WARNING: Watched path does not exist: /home/travis/build/mechmotum/mechmotum.github.io/content/images

WARNING: Empty alt attribute for image fiets2small.jpg in /home/travis/build/mechmotum/mechmotum.github.io/content/spring-2003.rst

WARNING: Empty alt attribute for image bike2small.jpg in /home/travis/build/mechmotum/mechmotum.github.io/content/spring-2003.rst

WARNING: Empty alt attribute for image Bike3xrl.jpg in /home/travis/build/mechmotum/mechmotum.github.io/content/spring-2003.rst

WARNING: Empty alt attribute for image Bike3xRev.jpg in /home/travis/build/mechmotum/mechmotum.github.io/content/spring-2003.rst

WARNING: Other images have empty alt attributes

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/bicycle-handling-qualities.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/computational-thinking.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/about.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/members.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/bicycle-parameters.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/index.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/sympy-beam.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/guide.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/human-powered-water-pump.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/ski-jump-safety.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/jobs.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/resonance.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/bicycle-data-logger.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/quadriplegic-friendly-tricycle.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/libretexts-jupyter.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/optimal-handling-bicycle.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/products.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/standing-control-id.rst: Ensure exactly one top level section

WARNING: Document title missing in file /home/travis/build/mechmotum/mechmotum.github.io/content/pages/research/rowing-performance.rst: Ensure exactly one top level section

WARNING: There is no "sortorder" attribute in "pages/jobs/determining-dynamics-perception-thresholds-of-bicycles.rst". Defaulting to slug order.

WARNING: There is no "sortorder" attribute in "pages/jobs/fast-musculoskeletal-simulations.rst". Defaulting to slug order.

WARNING: There is no "sortorder" attribute in "pages/jobs/how-fast-will-open-source-break.rst". Defaulting to slug order.

WARNING: There is no "sortorder" attribute in "pages/jobs/optimization-based-bicycle-design.rst". Defaulting to slug order.

WARNING: More files are missing the needed attribute.

WARNING: Unable to find '/implementation-of-ebike-speed-controller.rst', skipping url replacement.

WARNING: Unable to find 'ebike-speed-controller.rst', skipping url replacement.

[moorepants]

Some warning recorded from the CI today:

[10:31:19] WARNING  Defining PLUGIN_PATHS setting as string has been   log.py:91
                    deprecated (should be a list)                               
           WARNING  %s usage in CATEGORY_FEED_ATOM is deprecated, use  log.py:91
                    {slug} instead.                                             
[10:31:20] WARNING  Watched path does not exist: /home/runner/work/mec log.py:91
                    hmotum.github.io/mechmotum.github.io/content/image          
                    s                                                           
           WARNING  Empty alt attribute for image                      log.py:91
                    gazelle-steer-assist-bicycle.jpg in /home/runner/w          
                    ork/mechmotum.github.io/mechmotum.github.io/conten          
                    t/bicycle-of-the-future-postdoc.rst                         
           WARNING  Empty alt attribute for image ucd-kube-cluster.png log.py:91
                    in /home/runner/work/mechmotum.github.io/mechmotum          
                    .github.io/content/libretexts-wrapup.rst                    
           WARNING  Empty alt attribute for image                      log.py:91
                    libretexts-jupyterhub-login-page.png in /home/runn          
                    er/work/mechmotum.github.io/mechmotum.github.io/co          
                    ntent/libretexts-wrapup.rst                                 
           WARNING  Empty alt attribute for image                      log.py:91
                    libretexts-jupyter-guide.png in /home/runner/work/          
                    mechmotum.github.io/mechmotum.github.io/content/li          
                    bretexts-wrapup.rst                                         
           WARNING  Other images have empty alt attributes             log.py:91
[10:31:21] WARNING  There are 2 items "with slug "postdoctoral-researc log.py:91
                    her-opening-bicycle-of-the-future"" with lang en:           
                    /home/runner/work/mechmotum.github.io/mechmotum.gi          
                    thub.io/content/bicycle-of-the-future-postdoc.rst           
                    /home/runner/work/mechmotum.github.io/mechmotum.gi          
                    thub.io/content/bicycle-of-the-future-postdoc-2.rs          
                    t                                                           
           WARNING  There are 2 original (not translated) items with   log.py:91
                    slug "postdoctoral-researcher-opening-bicycle-of-t          
                    he-future":                                                 
                    /home/runner/work/mechmotum.github.io/mechmotum.gi          
                    thub.io/content/bicycle-of-the-future-postdoc.rst           
                    /home/runner/work/mechmotum.github.io/mechmotum.gi          
                    thub.io/content/bicycle-of-the-future-postdoc-2.rs          
                    t                                                           
[10:31:22] WARNING  There is no "sortorder" attribute in               log.py:91
                    "pages/jobs/active-bicycle-treadmill.rst".                  
                    Defaulting to slug order.                                   
           WARNING  There is no "sortorder" attribute in "pages/jobs/b log.py:91
                    icycle-balance-vestibular-contributions.rst".               
                    Defaulting to slug order.                                   
           WARNING  There is no "sortorder" attribute in               log.py:91
                    "pages/jobs/bicycle-crash-video-kinematics.rst".            
                    Defaulting to slug order.                                   
           WARNING  There is no "sortorder" attribute in               log.py:91
                    "pages/jobs/bobsleigh-simulator.rst". Defaulting            
                    to slug order.                                              
           WARNING  More files are missing the needed attribute.       log.py:91