intersphinx and introspection: fix the links

[jhpalmieri]

Ticket #9128 turned on the intersphinx extension for Sphinx, thus proving nice cross-referencing in the reference manual. In the notebook, though, this doesn't construct the correct links: executing

sage.algebras.group_algebra_new.GroupAlgebra.one_basis?

produces a docstring with a url

localhost:8000/FULL_PATH_TO_REFERENCE_MANUAL/sage/categories/algebras_with_basis.html#sage.categories.algebras_with_basis.AlgebrasWithBasis.ParentMethods.one_basis

Note that #13057 disabled intersphinx when using introspection because of a slow-down and apparent memory leak. Those issues need to be addressed — see #13127 — before this ticket can be merged. which should probably instead be

localhost:8000/doc/static/reference/sage/categories/algebras_with_basis.html#sage.categories.algebras_with_basis.AlgebrasWithBasis.ParentMethods.one_basis