ci: group react with react-dom in the docs ecosystem

[Jamyn]

react-dom peers react at its own exact version, so a PR bumping either one alone can never resolve. Dependabot opened exactly that twice — #186 (react-dom 19, react left at 18) and #187 (react 19, react-dom left at 18) — and both fail npm ci and docs-build.

Groups the two so they arrive as one PR. The group carries no update-types, so majors group too, and it is listed before docs-minor-and-patch because a dependency joins the first group it matches — otherwise that group would capture their minor updates and split the pair again.

Verified locally that the two applied together install and build clean: npm ci resolves, Docusaurus builds all 301 documents with onBrokenLinks: throw, and the docs tests pass. #186 and #187 can be closed once this merges.