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
Adds some guidance for designing clusters to be resilient to
failures, including example architectures.
Co-authored-by: James Rodewig <james.rodewig@elastic.co>
Previously, cross-cluster replication (CCR) documentation was located in
the Stack Overview:
https://www.elastic.co/guide/en/elastic-stack-overview/master/xpack-ccr.html
This adds CCR documentation to the Elasticsearch Reference Guide with a
level offset for headings.
The level offset and CCR Stack Overview docs will be removed in later
commits.
