This PR adds a user action to the SLM health indicator which checks each SLM policy's invocations
since last success field and reports degraded health (YELLOW) in the event that any policy is at or
above the failure threshold (default is 5 failures in a row).
Makes several changes to consolidate snapshot and backup-related docs.
Highlights:
* Adds info about supported ESS snapshot repository types
* Adds docs for Kibana's Snapshot and Restore feature
* Combines tutorial pages related to taking and managing snapshots
* Consolidates explanations of the snapshot process
* Incorporates SLM into the snapshot tutorial
* Removes duplicate "back up a cluster" pages