docs(chore): Switch to git sources, remove bundled extensions

why: Match tmuxp's gp-sphinx integration pattern — use git sources
pointing to git-pull/gp-sphinx#init-3 instead of local editable paths.
sphinx_fonts and sphinx_pytest_fixtures are now gp-sphinx packages.
what:
- Switch [tool.uv.sources] from local paths to git sources (init-3)
- Remove docs/_ext/sphinx_fonts.py (now sphinx-fonts package)
- Remove docs/_ext/sphinx_pytest_fixtures/ (now sphinx-autodoc-pytest-fixtures)
- Keep docs/_ext/spf_demo_fixtures.py (libtmux-specific badge demo)
- Re-add docs/_ext to sys.path for spf_demo_fixtures only

Author: tony