ci: align docs check Python version with ReadTheDocs (3.10)

RTD uses Python 3.10 per .readthedocs.yaml. CI was using 3.9, which
resolved different Sphinx/theme versions from uv.lock, masking build
failures that only appeared on RTD.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rtibblesbot

.github/workflows/check-docs.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Install uv
         uses: astral-sh/setup-uv@v7
         with:
-          python-version: 3.9
+          python-version: "3.10"
           activate-environment: "true"
           enable-cache: "true"
       - name: Install dependencies