chore(docs): settle yarn's unmet peer requirements - #25779

`yarn install` in docs/ printed YN0086 for 39 unmet peer requirements.
None is a dependency docs uses without declaring: react, react-dom,
@mdx-js/react and @docusaurus/plugin-content-docs are already direct
dependencies. packageExtensions in .yarnrc.yml now:

- mark optional the peers that no package loads where they go
  unprovided: Algolia autocomplete's client peers (.d.ts only),
  react on @docusaurus/types (types only) and on the helmet copies
  it holds for types, core's @mdx-js/react (core resolves it from the
  site directory), file-loader's webpack and ts-node's @types/node;
- pass a real peer through from the parent where an intermediate
  dropped it: plugin-content-docs via theme-mermaid, and react and
  react-dom via plugin-css-cascade-layers.

Each entry is pinned to the version it was checked against, so an
upgrade brings the warning back. No package.json or yarn.lock change:
a clean install resolves the same tree as before.