chore(triagebot): add [no-mentions] and [note] (#15517)

`[no-mentions]` is for preventing something like
<#15378 (comment)>.
Well, maybe not working for PR description, I don't know.

`[note]` is for somebody else wanting to help summarize but having no
permission to edit top-level comments.

See docs for more:

* https://forge.rust-lang.org/triagebot/note.html
* https://forge.rust-lang.org/triagebot/no-mentions.html

Author: epage

triagebot.toml

@@ -26,8 +26,17 @@ label = "O-windows"
 
 [shortcut]
 
+# Enable issue transfers within the org
+# Documentation at: https://forge.rust-lang.org/triagebot/transfer.html
 [transfer]
 
+# Prevents mentions in commits to avoid users being spammed
+[no-mentions]
+
+# Enable `@rustbot note` functionality
+# Documentation at: https://forge.rust-lang.org/triagebot/note.html
+[note]
+
 [merge-conflicts]
 remove = []
 add = ["S-waiting-on-author"]