docs: Move enum type documentation to Markdown

Helps: #3037

docs/reference/gobject/gobject.toml.in

@@ -43,6 +43,7 @@ urlmap_file = "urlmap.js"
 # The same order will be used when generating the index
 content_files = [
   "boxed.md",
+  "enum-types.md",
 ]
 content_images = [
 ]