Adopt the unified Astropy theme, or elements thereof #140

Hi all,

I'm opening this to start a discussion, I appreciate some of the thoughts here are a big lift.

More context is here: astropy/astropy-project#522

I'm working on rolling out a unified theme to the astropy home page and all the coordinated packages. There are two objectives of doing this:

1. A unified navigation for all these sites, so you can for example jump between astropy.org/code-of-conduct and the API documentation for astropy.wcs seamlessly.
2. Leverage the Read the Docs server side search so that you can search between all of these sites (test it out here).

I'd love to have https://learn.astropy.org join in on this, and I know you've spent ages thinking about search in the past.

To get the full experience, we'd need to migrate learn.astropy.org to be a sphinx project and be hosted on Read the Docs. In the short term, or if that's too much of a lift perhaps we could restyle learn.astropy.org and change the top nav so that it visually and navigation wise fits in with the rest of the sites?

One other stand-alone thing that would be nice to make work is the unified search with astropy.org. To make this work we would only have to host astropy learn on read the docs rather than github pages (no other build changes), and then port some js from the sunpy sphinx theme.