Move the notifications documentation to the "Configuration" section.
Summary: Notifications isn't really an application, this documentation should probably be moved to the "Configuration" seciton.
Test Plan: N/A
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9402