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