ci(docs): install latest pnpm via corepack to fix docs deploy
The Deploy Docs workflow failed at pnpm/action-setup@v6: with a floating
`version: 11` the action lays down a bundled pnpm and then self-updates to the
latest 11.x, and that switch crashes ("Cannot use 'in' operator to search for
'integrity' in undefined" in lockfileToDepGraph), so the build job never
reaches the site build.
Replace pnpm/action-setup with corepack (bundled with Node), which installs the
latest pnpm cleanly, and cache the pnpm store via actions/cache. Always tracks
the latest pnpm release.
Verified locally with pnpm 11.12.0: `pnpm install --frozen-lockfile`, `pnpm
build`, `pnpm run html-validate` (0 errors) and `pnpm run pa11y` (10/10 URLs, 0
errors) all pass.