Bump actions/setup-python from 5 to 6

Bumps actions/setup-python from 5 to 6 in GitHub Actions workflows. Resolved a merge conflict with main's separate actions/checkout v5→v6 bump by combining both updates.

Author: dependabot[bot]

.github/workflows/docs.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v6
         with:
           python-version: "3.12"
       - run: pip install mkdocs-material mkdocstrings[python]