fix(docs): bump Python to 3.10 in docs CI to fix empty API pages (#937)

Root cause: The docs CI uses Python 3.9, which caps torch at ~2.6. These
older torch versions are incompatible with modern setuptools>=82 (which
removed pkg_resources), causing import torch.utils.cpp_extension to
fail. Since _backend.py is imported at module level via
_make_lazy_cuda_cls() in _wrapper.py, this crashes the entire import
gsplat chain. Sphinx catches the error silently and deploys empty pages.

Author: liruilong940607

.github/workflows/doc.yml

@@ -21,7 +21,7 @@ jobs:
 
       - uses: actions/setup-python@v3
         with:
-          python-version: '3.9'
+          python-version: '3.10'
 
       - name: Install dependencies
         run: |