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.
