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)

Lisez la proposition de Cycle 5 dans finance/proposal-calls/cycle5/aperio-documentation.md et inspectez le build actuel d’astropy.org pendant que la migration Sphinx est en cours. Définissez le périmètre de l’hébergement et de la migration, puis vérifiez que le site est hébergé sur RTD et disponible pour la recherche unifiée via la RTD search API.

Rédigé par le modèle d'indexation à partir du texte de l'issue.