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 https://github.com/sphinx-doc/sphinx/issues/11722#issuecomment-1886157588