Add documentation on how to extend the theme

This is something that I get questions on a lot:

• New maintainers want to know how to add CSS/JS. We prescribe adding in a setup() extension point via app.add_stylesheet() to get around our extension
• Companies doing a deeper conversion of our theme to match their brand should know how to extend our templates
• Our template blocks should be documented some? Or at least we point to the Sphinx default list of blocks that we emulate