Manual: compatibility fix for Sphinx 8

In Sphinx 8, returned filesystem path objects are no longer extended to have
implicit string conversion methods. The conversion to a string must now be made
explicitly.

See this entry in the changelog for Sphinx 8.0:

 #12650, #12686, #12690: Extend the deprecation for string methods on
 :py:class:`~pathlib.Path` objects to Sphinx 9. Use :py:func:`os.fspath` to
 convert :py:class:`~pathlib.Path` objects to strings, or
 :py:class:`~pathlib.Path`'s methods to work with path objects. Patch by Adam
 Turner.