Missing docs on attaching alert groups to each other

[alyssawada]

Background

OnCall docs lack documentation describing the functionality of attaching one alert group to another.

tl;dr This issue was raised in a demo where the need for grouping incidents together was expressed. See Slack thread for additional context

What needs to be done

Add documentation explaining the functionality of attaching alert groups to each other.

1. Clarification needed: Determine intended use and whether it was intentionally undocumented.
2. Use case exploration: Should cover applications and some guidance on how to utilize it.