Do not use locally hosted Lato or Roboto fonts

The following fonts use *a lot* of our bandwidth. It'd be great if we could just point to the Google's servers and the hosted version of those fonts.

* /_static/fonts/Lato-Regular.ttf
* /_static/fonts/Lato-Bold.ttf
* /_static/fonts/RobotoSlab-Bold.ttf
* /_static/fonts/RobotoSlab-Regular.ttf

Based on the issue at https://github.com/rtfd/sphinx_rtd_theme/issues/611, here are some options...
* Override @font-face CSS and point to Google's hosted fonts
* Have a body CSS that uses some common sans serif font
* Upgrade to a new version of the theme that might be lighter weight? https://github.com/rtfd/sphinx_rtd_theme/pull/639

Start by locating the listed font files under /_static/fonts/ and the CSS references that load them. Choose one of the proposed approaches, then verify that the documentation site no longer loads those local fonts and that the Sphinx documentation still builds correctly.

Written by the indexing model from the issue text.