docs(chore): Remove bundled extension files and tests

why: sphinx_fonts, sphinx_pytest_fixtures, and their tests now live in
the gp-sphinx workspace as independent packages. The badge demo page
depended on spf_demo_fixtures which was libtmux-specific scaffolding.
what:
- Remove docs/_ext/spf_demo_fixtures.py and remaining __pycache__
- Remove tests/docs/_ext/ entirely (tests migrated to gp-sphinx)
- Remove docs/demo/ badge showcase pages
- Remove demo/index from docs toctree
- Remove dead mypy overrides for sphinx_fonts and sphinx_autodoc_pytest_fixtures
- Remove _ext sys.path from docs/conf.py

Author: tony

docs/_ext/spf_demo_fixtures.py

@@ -15,7 +15,6 @@
 project_src = project_root / "src"
 
 sys.path.insert(0, str(project_src))
-sys.path.insert(0, str(cwd / "_ext"))  # spf_demo_fixtures for badge demo page
 
 # package data
 about: dict[str, str] = {}

docs/conf.py

@@ -105,5 +105,4 @@ project/index
 history
 migration
 glossary
-demo/index
 ```

docs/demo/index.md

@@ -118,13 +118,6 @@ files = [
   "tests",
 ]
 
-[[tool.mypy.overrides]]
-module = ["sphinx_fonts"]
-ignore_missing_imports = true
-
-[[tool.mypy.overrides]]
-module = ["sphinx_autodoc_pytest_fixtures", "sphinx_autodoc_pytest_fixtures.*"]
-ignore_missing_imports = true
 
 [tool.coverage.run]
 branch = true