Add documentation for Ruler HA

[yeya24]

Follow up of #6129.

Add a doc for Ruler HA (both API HA and evaluation HA) to https://cortexmetrics.io/docs. Either https://cortexmetrics.io/docs/guides/ or https://cortexmetrics.io/docs/operations/.

The doc should cover under which scenario HA is recommended, relevant configurations and config tuning.

[yeya24]

#6129 (review)

Make sure we also mark those flags as experimental.

[stale]

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.

[yeya24]

@rajagopalanand We should revisit this to provide a user guide doc for Ruler HA and rule API HA