[BUG] Update notifications plugin documentation

[AWSHurneyt]

The notifications plugin documentation (link) is missing some key information, and needs updating.

1. The permissions list page doesn't list any of this plugin, or its available permissions - https://opensearch.org/docs/latest/security/access-control/permissions/
2. The documentation doesn't have a page that explains the various plugin settings (e.g., filter_by_backend_roles). This is an example of a settings page - https://opensearch.org/docs/latest/observing-your-data/alerting/settings/