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)

finance/proposal-calls/cycle5/aperio-documentation.md の Cycle 5 提案を読み、Sphinx の移行が進行中の astropy.org の現在のビルドを調査してください。ホスティングと移行の範囲を定義し、そのうえでサイトが RTD でホストされ、RTD search API を通じて統合検索から利用できることを確認してください。

索引モデルが issue の本文から書いたものです。