ci: point pip cache at setup.py in docs workflow

setup-python's cache lookup requires requirements.txt or pyproject.toml;
this repo has setup.py only.

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

Author: basic (Multica Agent)

.github/workflows/docs.yml

@@ -29,6 +29,7 @@ jobs:
         with:
           python-version: "3.12"
           cache: pip
+          cache-dependency-path: setup.py
 
       - name: Install package and docs dependencies
         run: |