Migrate astropy.org to be hosted RTD

This is a early warning / heads up issue.

As part of my Cycle 5 proposal to improve the unification of the astropy docs (https://github.com/astropy/astropy-project/blob/b04f81d10e268f700b4d6b9fe1fac5811daf155b/finance/proposal-calls/cycle5/aperio-documentation.md) it would be helpful to migrate astropy.org to be hosted on RTD.

The main motivation for this is that it allows the site to be indexed by the RTD server side search and to access that API without any tickery. This allows the unified search to work across the various projects *and* the homepage.

(It should also be noted that I'm in the process of migrating the site to be built with sphinx to enable all this as well)

Read the Cycle 5 proposal at finance/proposal-calls/cycle5/aperio-documentation.md and inspect the current astropy.org build while the Sphinx migration is in progress. Define the hosting and migration scope, then verify that the site is hosted on RTD and available to the unified search through the RTD search API.

Written by the indexing model from the issue text.