Set up readthedocs

[thedrow]

• Create sphinx configuration.
• Enable readthedocs.
• Create a subdomain.
• Ensure HTTPS works properly.
• Add a link to Celery's documentation that points to that subdomain.

[thedrow]

master does not build yet.
Once #19 is merged it will be.

[jheld]

Is #17 related to this or a downstream consequence once this is finished?

[thedrow]

Unrelated. I just think we should have a better website to celebrate the release of Celery 5.

[ruslankrivoshein]

Any moves?