Update Documentation for Store Gateway

[justinrush]

Is your feature request related to a problem? Please describe. The configuration documentation for the store gateway seems to be the only source of information on this tier. From that documentation it isn't clear how we scale that stateful set in kubernetes. It seems to take a similar cluster-like config as other tiers - kvstore, memberlist, etc - however, some of the help text makes me think its purpose is for sharding data across multiple backend storage services.

Describe the solution you'd like It would be helpful if there were information about what would cause me to want to scale this service and some information that helps me determine the # of replicas, resources requests/limits, and appropriate kvstore target.

[stale[bot]]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.