ci(docs): use actions/setup-python so the generator runs on any runner image

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/docs.yml

@@ -23,6 +23,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.x'
+
       - name: Generate the documentation site
         run: |
           python3 --version