[gcp-sphinx-docfx-yaml] Fix Sphinx 1.5.5 / Jinja2 3.1 deadlock in docs session

[ohmayr]

The docs session is postponed because the required sphinx-docfx-yaml extension is hard-pinned to Sphinx 1.5.5, which breaks on modern Jinja2 and MarkupSafe. We need to either pin the legacy environment or modernize the extension to support Sphinx 4.5+.