PIPE-3211: prune obsolete and unused documentation dependencies
- Remove legacy Python 2/3 compatibility shim future and redundant transitive dependency urllib3 from docs optional-dependencies.
- Remove dead PyPI dummy package nodejs (Node.js is provided by Conda).
- Remove unused Sphinx extensions (nbsphinx, sphinx-autoapi), unused theme (sphinx_rtd_theme), and ipympl widget not active in conf.py.
- Update documentation list in `docs/source/dependencies.md`.
- Synchronize and prune `pixi.lock` across docs environments.
