lint *.markdown files also

Author: jay7x

.github/workflows/markdownlint.yml

@@ -21,5 +21,5 @@ jobs:
       - uses: actions/checkout@v6
       - uses: DavidAnson/markdownlint-cli2-action@v22
         with:
-          globs: '**/*.md'
+          globs: '**/*.{md,markdown}'
         continue-on-error: true # Allow this job to fail for now