Document how mimir alert manager works

[Rotfuks]

Motivation

In order for others inside and outside our team to understand how the mimir alertmanager works, we need to make sure that we document it in the intranet.

Todo

• Create a documentation of the Mimir Alertmanager in https://intranet.giantswarm.io/docs/observability/
  • Make sure this documentation includes a general "how it works", including an architecture diagram in the context of our observabtility platform
  • Make sure the documentation includes a general "how to use it", explaining the basic interactions with the alert manager, maybe linking to the mimir alertmanager docs

Outcome

• We have documented and shared the knowledge about the mimir alertmanager