chore(mypy[docs]) Include docs lexers in default type checks

why: The docs lexer tests import modules from docs/_ext, but the default mypy scope only covered src and tests so uv run mypy failed on unresolved imports and reported a stale docutils override.
what:
- add docs/_ext to the default mypy files list
- remove the unused docutils override entries
- keep the existing shtab and pygments import exceptions

Author: tony

pyproject.toml

@@ -139,14 +139,13 @@ warn_unused_configs = true
 files = [
   "src",
   "tests",
+  "docs/_ext",
 ]
 strict = true
 
 [[tool.mypy.overrides]]
 module = [
   "shtab",
-  "docutils",
-  "docutils.*",
   "pygments",
   "pygments.*",
 ]