Document how CCR may be used to speed up indexing. (#52717)

One architecture that we have recommended to several users to speed up
indexing involved using CCR to prevent searching from stealing resources
from indexing.

docs/reference/how-to/indexing-speed.asciidoc

@@ -128,6 +128,15 @@ The default is `10%` which is often plenty: for example, if you give the JVM
 10GB of memory, it will give 1GB to the index buffer, which is enough to host
 two shards that are heavily indexing.
 
+[float]
+=== Use {ccr} to prevent searching from stealing resources from indexing
+
+Within a single cluster, indexing and searching can compete for resources. By
+setting up two clusters, configuring <<xpack-ccr,{ccr}>> to replicate data from
+one cluster to the other one, and routing all searches to the cluster that has
+the follower indices, search activity will no longer steal resources from
+indexing on the cluster that hosts the leader indices.
+
 [float]
 === Additional optimizations