triagebot: add some backlinks to triagebot docs

Author: jieyouxu

triagebot.toml

@@ -1,4 +1,15 @@
+# This file's format is documented at
+# https://forge.rust-lang.org/triagebot/pr-assignment.html#configuration
+
+# ------------------------------------------------------------------------------
+# Labels
+# ------------------------------------------------------------------------------
+
 [autolabel."pr-not-reviewed"]
 new_pr = true
 
+# ------------------------------------------------------------------------------
+# PR assignments
+# ------------------------------------------------------------------------------
+
 [assign]