Document how canonical domains work

[humitos]

We have implemented the redirects on canonical domains (I didn't find the PR). In this comment, we mentioned how they work #8250 (comment). However, our documentation doesn't explain how canonical domains work and nor their redirects

• https://docs.readthedocs.io/en/stable/automatic-redirects.html
• https://docs.readthedocs.io/en/stable/guides/canonical.html

We should update our documentation to reflect this because it's generating confusion to our users.

[humitos]

When working on this issue, we need to mention that it's required to re-build old versions when the Canonical URL is updated (from #8533 (comment))

[stsewd]

That is already mentioned in the page https://docs.readthedocs.io/en/stable/canonical-urls.html.

[benjaoming]

From the issue description:

However, our documentation doesn't explain how canonical domains work and nor their redirects

Now since #9676, the docs explain:

Your documentation can have multiple secondary domains but only one canonical domain name. Additional domains or subdomains will redirect to the canonical domain.

I believe this should be sufficient. But please LMK if you had something else in mind 👍