[Images] Add details to supported ANS notification types

[dochne]

Summary

Cloudflare Images now supports two additional notifications:

• Image Notifications - this allows customers to get notifications when a user uses Direct Upload on success/failure
• Image Transformation Notifications - this gives users a notification when they've exceeded their the free tier limits for transformations.

This adds the appropriate documentation.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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