Add documentation about high availability

[ultrotter]

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