workflows/docs: Check for broken links, use existing rake test task

- We had `rake test` as a task, but we never actually ran it in CI or
  anywhere before building the site.
- This will make things slower (or maybe it's taking ages because of my
  dodgy hotel internet connection), maybe we should only run it on a
  schedule as a separate job?
- Fixes https://github.com/Homebrew/brew/issues/ 15908.

.github/workflows/docs.yml

@@ -40,6 +40,10 @@ jobs:
         working-directory: ${{ steps.set-up-homebrew.outputs.repository-path }}/docs
         run: bundle exec rake lint
 
+      - name: Check for broken links in docs
+        working-directory: ${{ steps.set-up-homebrew.outputs.repository-path }}/docs
+        run: bundle exec rake test
+
       - name: Build docs site
         working-directory: ${{ steps.set-up-homebrew.outputs.repository-path }}/docs
         run: bundle exec rake build