This provides a simple unlisted special page,
Special:DisplayNotificationsConfiguration, to document how the
notifications are structured and configured. It shows:
* Which types are in each category
* Which notify types (web/email) can be enabled for each category
* Which notify types are mandatory for each category
* Which notify types are enabled by default:
** For existing users
** For new users
Bug: T132127
Change-Id: I25b447a69a7c984941dfd703345d7977c0000bfe
