ci: run docs:build on PRs to catch dead links

[suleimansh]

The VitePress docs site (@gemstack/docs, added in #58) is not covered by CI. pnpm build / typecheck / test skip it because its scripts are docs:* only.

Add a step (or job) that runs pnpm --filter @gemstack/docs docs:build on pull requests, so a dead internal link or a broken page fails CI before merge instead of being caught only by a local build.

Low priority: until then, run pnpm --filter @gemstack/docs docs:build locally before pushing docs changes.