Summary

In this PR, we added a new tab to the alert details page to show related alerts. This ticket is a follow-up to that work for implementing the following improvements:

1. Showing the number of active alerts in the tab
2. Selecting Rule name as the default group by and opening the first group
3. [Maybe] adding an event timeline to this tab (related to https://github.com/elastic/observability-dev/issues/3925)