Add module (root) reference

[buhtz]

Might fix #1352

Solution was proposed by Sphinx maintainers them self. https://github.com/orgs/sphinx-doc/discussions/12027#discussioncomment-8656546

Now your objects.inv contain the right reference

$ python3 -m sphinx.ext.intersphinx objects.inv | grep module
py:module
	docx                                     index.html#module-docx

It is up to you to decide if you name it "python-docx" (name of distribution package) or "docx" (name of import package). From a users perspective I would prepare to have it named as the import package docx.