Update High Availability Section of Gitlab Architecture Document

The Gitlab chart comes with a handful of HorizontalPodAutoscaler resources by default, which make use of the metrics.k8s.io APIService to scale up and down deployments.

Ensure the High Availability section of the Gitlab Architecture doc references the presence of these HorizontalPodAutoscalers and that every pod/deployment/statefulset in the Gitlab chart can be ran with more than one replica out of the box with the exception of gitaly. Which has some (BigBang/P1) un-tested prerequisites to run as a cluster with High Availability.

For Gitaly we have outlined in the docs/production.md document that bumping up the resources a fair amount is enough to cover for this lack of HA support.