fix(ci): drop pip cache from docs workflow (#35)

setup-python@v6 hard-fails when cache:pip finds no requirements.txt or
pyproject.toml to hash (v5 only warned). The docs job installs
mkdocs-material inline, so there is nothing to cache — remove the cache
key.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: itaywol

.github/workflows/docs.yml

@@ -28,7 +28,6 @@ jobs:
       - uses: actions/setup-python@v6
         with:
           python-version: "3.x"
-          cache: pip
 
       - name: Install MkDocs Material
         run: pip install mkdocs-material