Add beta/develop version to ReadTheDocs

I would love to check the docs of the “beta/alpha/next” 9.0 branch. Also currently the states that it is the documentation for 8.2.

Made 8.3 the “latest” and unhid 9.0 (at was already rendered to Neos 9.0 Documentation — Neos CMS 9.0.x documentation, just not visible in the version selector).

Give the existing renderings some time to update, and that all should iron itself out.