Document Snaps notifications

[alexandratran]

Description

Add feature page for Snaps notifications.

Issue(s) fixed

Fixes #1261

Preview

https://docs.metamask.io/1261-notifications/snaps/features/notifications/

Checklist

Complete this checklist before merging your PR:

• [x] If this PR contains a major change to the documentation content, I have added an entry to the top of the "What's new?" page.
• [ ] The proposed changes have been reviewed and approved by a member of the documentation team.

[github-actions[bot]]

Preview published: 1261-notifications

[github-actions[bot]]

Preview published: 1261-notifications

[github-actions[bot]]

Preview published: 1261-notifications