Add docs-lint to CI

make lint runs docs-lint before golangci-lint, but CI only runs golangci-lint directly via the GitHub Action — it never calls make lint or hack/docs-lint.sh. The docs workflow only builds for deployment, no linting.

This means docs issues like stale sidebar entries (e.g. the current miren-cloud entry pointing at nothing) go undetected until someone runs make lint locally. We should add a docs-lint step to the test workflow so these get caught in CI.