Add triagebot mention for tidy ui_suites changes

Author: reddevilmidzy

triagebot.toml

@@ -1204,6 +1204,12 @@ cc = ["@davidtwco", "@wesleywiser"]
 message = "`tidy` extra checks were modified."
 cc = ["@lolbinarycat"]
 
+[mentions."src/tools/tidy/src/ui_suites.txt"]
+message = """
+`tests/ui/` directories have been modified. Please add or update docs for the
+new or modified directories in `tests/ui/README.md`.
+"""
+
 [mentions."src/bootstrap/src/core/config"]
 message = """
 This PR modifies `src/bootstrap/src/core/config`.