Fix none type link

[Kirubapanneers]

This PR restores the hyperlink for None in type annotations, which stopped working after Sphinx 4.4.0.

The previous commit (5da68c3) removed special handling for None. I added that logic back in PyField and PyTypedField classes. None should be handled as an object reference (:obj:) not as a class reference (:class:).

##Testing
I ran all Python domain tests (98 tests passed).
I created a small test document using None in type hints.
Verified that None is now clickable and properly linked again.

Note on failing CI checks

The following test failures appear to be pre existing issues unrelated to this PR:

• test_autogen
• test_autogen_remove_old
• test_numfig_disabled_warn

These tests are failing on the main branch as well and are not caused by the changes in this PR.

##My changes only touch:

• sphinx/domains/python/_object.py - restored None hyperlink handling
• sphinx/util/typing.py and doc/index.rst - fixed formatting issues

References
fixes #10899