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)

Lies den Vorschlag für Cycle 5 unter finance/proposal-calls/cycle5/aperio-documentation.md und prüfe den aktuellen Build von astropy.org, während die Sphinx-Migration läuft. Definiere den Umfang des Hostings und der Migration und verifiziere anschließend, dass die Website auf RTD gehostet wird und über die RTD search API für die einheitliche Suche verfügbar ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.