test(docs/_ext): Remove upstream extension tests now covered by PyPI packages

why: sphinx-fonts and sphinx-argparse-neo are now standalone PyPI packages
     (0.0.1a0). Their test suites live upstream; tmuxp should not duplicate
     or maintain them.
what:
- Remove tests/docs/_ext/test_sphinx_fonts.py
- Remove tests/docs/_ext/sphinx_argparse_neo/ (7 test files)
- Remove tests/docs/_ext/test_argparse_{exemplar,lexer,roles}.py
- Remove tests/docs/_ext/test_cli_usage_lexer.py
- Remove tests/docs/__init__.py and supporting conftest/init files

Author: tony