chore: add markdownlint to extensions.json

.vscode/extensions.json

@@ -2,6 +2,7 @@
   "recommendations": [
     "streetsidesoftware.code-spell-checker",
     "streetsidesoftware.code-spell-checker-french",
-    "redhat.vscode-yaml"
+    "redhat.vscode-yaml",
+    "davidanson.vscode-markdownlint"
   ]
 }