* Creates a prerequisites section in the cross-cluster replication (CCR)
overview.
* Adds concise definitions for local and remote cluster in a CCR context.
* Documents that the ES version of the local cluster must be the same
or a newer compatible version as the remote cluster.
This commit expands the ccr overview page to include more information
about the lifecycle of following an index. It adds information linking
to the remote recovery documentation. And describes how an index can
fall-behind and how to fix it when this happens.
This commit is our first introduction to cross-cluster replication
docs. In this commit, we introduce the cross-cluster replication API
docs. We also add skelton docs for additional content that will be added
in a series of follow-up commits.
