Apply new RTD theme options

Applied theme changes that are configurable without modifying CSS or
HTML. Deeper changes to the theme can always be investigated later.

Changes of note:

* "+" symbol in sidebar next to sections which can be expanded to show
  subsections. We may wish to investigate changing the color to increase
  visibility, if this is an option we want to add to the docs.

* Background color of sidebar updated to match website

* External links are styled, so as to indicate such.

QA steps: viewed rendered html in browser, ran linkcheck.

No associated issue, minor changes to test out.

Signed-off-by: Katrina Prosise <[email protected]>

source/conf.py

@@ -299,7 +299,11 @@
 # than canonical_url
 html_theme_options = {
     'logo_only': True,
-    'canonical_url': 'https://docs.foundries.io/latest/'
+    'canonical_url': 'https://docs.foundries.io/latest/',
+    'collapse_navigation': False,
+    'sticky_navigation': False,
+    'style_external_links': True,
+    'style_nav_header_background': '#2C1256',
 }