chore: Upgrade theme to 1.9: Sphinx 9, uv, Python 3.11+, Node 22

This release upgrades the core toolchain: Sphinx 9.x, myst-parser 5.x, Python 3.11+, Node.js 22, and replaces Poetry with uv for dependency management.

The bundled sphinx-multiversion extension has been updated for Sphinx 9 compatibility.

1. Install uv first:

make setupenv

2. Then set up the environment and build the docs:

make setup
make preview

We'll help projects migrate from 1.8 to 1.9. See the upgrade guide for step-by-step instructions.

Author: dgarcia360