Simple sphinx settings to improve navigation

[cbkerr]

Small fix to improve navigability of the docs while large scale projects like #64 continue.

Motivation

I always felt lost in the docs, particularly by not knowing the hierarchy of items in the sidebar.

Details

With these settings:

• show a plus sign when mousing over headings in the sidebar to avoid having to click to load the page to expand the subtree
• show navigation buttons at top and bottom of page because lots of our pages are long

The options are described here.

Before

After

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My code follows the code style guideline of this project.

[atravitz]

looks good!

[pepak13]

Nice Feature!