Proxito: document custom prefixes feature

[stsewd]

What's the problem this feature will solve?

Custom prefixes for projects and subprojects were implemented in #10156, but we never documented them publicly. We have some internal docs about it in https://github.com/readthedocs/readthedocs.org/blob/main/readthedocs/proxito/README.rst#custom-path-prefixes.

We mention this feature in our pricing page (https://about.readthedocs.com/pricing/).

URL customization - Fine tune URLs used for your projects

We should link to our documentation from there with all the use cases this feature solves (proxying, removing or changing the "/projects" prefix).

There is also the discussion if we should expose this to users or manually handle this via support.

[agjohnson]

Yeah, I wouldn't mind exposing control of this as a beta feature/admin page as we are documenting it. Controlling this on behalf of users is tricky and requires scheduling changes and some back and forth. If users have this control they can remedy problems quicker.