Spell check .md and .mdx files (#41398)

Co-authored-by: Julien Déramond <[email protected]>

Author: Jason3S

.github/workflows/cspell.yml

@@ -31,6 +31,6 @@ jobs:
         uses: streetsidesoftware/cspell-action@v6
         with:
           config: ".cspell.json"
-          files: "**/*.md"
+          files: "**/*.{md,mdx}"
           inline: error
           incremental_files_only: false