Send email notifications for Generic Alerts docs

What does this MR do?

This adds documentation on the emails we send when we receive an alert (either Prometheus of a generic alert).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #207488 (closed)