Docs: document our CORS rules for docs pages

[stsewd]

What's the problem this feature will solve?

We recently changed our CORS header for documentation files, we don't have any of this documented.

Describe the solution you'd like

We should document how our CORS rules work for docs pages, security implications (none, at least users are doing something custom proxying to us or similar). Similar to what we have in https://docs.readthedocs.io/en/stable/api/cross-site-requests.html.