file_icons: Use a separate icon key for Markdown files (#24648)

This PR updates the file icon mappings such that Markdown (`.md`,
`.markdown`) files map to the `markdown` key.

Release Notes:

- Icon themes: Added the ability to change the file icon for Markdown
(`.md`, `.markdown`) files.

assets/icons/file_icons/file_types.json

@@ -113,8 +113,8 @@
     "lua": "lua",
     "m4a": "audio",
     "m4v": "video",
-    "markdown": "document",
-    "md": "document",
+    "markdown": "markdown",
+    "md": "markdown",
     "mdb": "storage",
     "mdf": "storage",
     "mdx": "document",