Rebuilding the documentation theme

The Vanilla theme used in the documentation is based on vanilla-sphinx-test. All the source files needed to generate it are in the docs/vanilla/ directory, though for simplicity the output of building the theme’s CSS is currently committed to git rather than built dynamically. To rebuild it, run the following on at least Ubuntu 22.04 in a container with yarnpkg installed:

(cd docs && yarnpkg && yarnpkg run build-css)