Provide a better error message if the master document is not included in project documents

[AA-Turner]

If index.rst does not appear in the pattern of a custom {include,exclude}_patterns, the error message is less than helpful. Instead, we should check that the master doc appears in the Project documents, and provide a better error if it doesn’t.

Inspiration from #11722 (comment)

[aflip]

I could take this up, but it is unclear to me how I would do ", we should check that the master doc appears in the Project documents, and provide a better error if it doesn’t."

[picnixz]

See #12011 for an implementation and some directions I suggested. I wanted to add tests on that PR but the author closed it before I had the time to (I forgot about the PR and since they closed it, I assumed they did not want to work on it anymore).

(Since people were a bit confused about how to do the implementation, I removed the 'easy' label but don't be scared. It's actually not 'easy' yet it is not 'hard'. However, you should update yourself with my comments on #12011).

[picnixz]

So, the PR #12011 is now back on track (and thus the task is taken).