Migrate (old site) to RTD

[Cadair]

This PR is just to migrate the current website to be built (basically a no-op) and hosted by RTD, so we can do the migration without any interruption in service or change in site.

You can see a successful build here: https://app.readthedocs.org/projects/astropy-org-test/builds/31273181/
and preview here: https://astropy-org-test.readthedocs.io/old-rtd-migration/

This PR can be merged without any trouble (it just adds the RTD config file), then the following steps need to happen:

• Create a RTD project (or rename and use my test one)
• Configure custom domains for that RTD project, and point the astropy.org DNS at RTD as instructed.
• Then we can merge Rewrite to a sphinx project and use unified theme #698 once we are happy with it, without any extra steps or worry.

[Cadair]

@eteq As purported keeper of the holy DNS of Antioch I guess this is on you.

[pllim]

Magical! I don't think this needs DNS blessing?

[Cadair]

It can be merged as-is, without any ill effects and then the RTD setup and DNS can be done when someone gets the time.

[pllim]

RTD should be up: https://astropy-org.readthedocs.io/en/latest/

[pllim]

I added the same people I see for astropy core RTD. If different, please let me know.