Mention devtest in frontend contributing guidelines (#26249)

Our `devtest` page contains some good UI examples which should be taken
as example when working on Gitea UI. Mention it in our frontend
contributing guidelines to for new contributors to know about it.

Author: denyskon

docs/content/contributing/guidelines-frontend.en-us.md

@@ -133,3 +133,7 @@ A lot of legacy code already existed before this document's written. It's recomm
 ### Vue3 and JSX
 
 Gitea is using Vue3 now. We decided not to introduce JSX to keep the HTML and the JavaScript code separated.
+
+### UI Examples
+
+Gitea uses some self-made UI elements and customizes others to integrate them better into the general UI approach. When running Gitea in development mode (`RUN_MODE=dev`), a page with some standardized UI examples is available under `http(s)://your-gitea-url:port/devtest`.