fix: add doc8 to doc dependency group, remove twine duplicate

tox -e docs runs doc8 to lint RST files, but doc8 was missing from the
doc group causing a command-not-found error. Also removes twine from
doc (it belongs only in quality).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: farhan

pyproject.toml

@@ -71,9 +71,9 @@ quality = [
 doc = [
     {include-group = "test"},
     "build",
+    "doc8",
     "Sphinx",
     "sphinx-book-theme",
-    "twine",
 ]
 
 # From requirements/ci.in