`docs.yml`: sphinx build on PRs + main pushes (via `uv sync
--no-dev --group docs`), deploying to gh-pages only from main
pushes using the official
`actions/{upload-pages-artifact,deploy-pages}` pair - the
`msgspec`-style flow wished for in #123. No `d2` bin in CI (yet) so
the committed SVGs serve as-is.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi