Customize the HTML theme with natcap branding

[emlys]

The alabaster theme has a lot of configuration options: https://alabaster.readthedocs.io/en/latest/customization.html#customization

It might be nice to incorporate natcap colors and fonts.

We could also fix some styling issues:

• Lots of words are split across lines
• The sidebar isn't wide enough for some model names
• Tables should be in a different font

[emlys]

@jagoldstein also suggested:

would it be possible to lock the TOC as a panel on the left so that it doesn't scroll out of view as users move down a page?

https://github.com/natcap/invest.users-guide/pull/95#issuecomment-1310993510