ci: add missing markdownlint CI step

The CI step that runs markdown lint accidentally got removed as part of
PR updates. Add the CI step back in.

Signed-off-by: Blaine Gardner <blaine.gardner@ibm.com>

.github/workflows/docs-check.yml

@@ -36,6 +36,10 @@ jobs:
         with:
           python-version: 3.9
 
+      - uses: DavidAnson/markdownlint-cli2-action@v16
+        with:
+          globs: 'Documentation/**/*.md",!Documentation/Helm-Charts'
+
       - name: Check helm-docs
         run: make check-helm-docs
       - name: Check docs