Add documentation about high availability

This summarizes the principles for HA as stated by Ben, an old blog post by Julius
explaining how it works[1], and the current implementation.

Signed-off-by: Guido Trotter <[email protected]>

[1] https://promlabs.com/blog/2023/08/31/high-availability-for-prometheus-and-alertmanager-an-overview/#ha-for-the-alertmanager

Author: Guido Trotter