[DOCS] Add 'remote clusters' requirement link to CCR docs (#47185)

Author: ppf2

docs/reference/ccr/overview.asciidoc

@@ -17,6 +17,7 @@ Replication is pull-based. This means that replication is driven by the
 follower index. This simplifies state management on the leader index and means
 that {ccr} does not interfere with indexing on the leader index.
 
+IMPORTANT: {ccr-cap} requires <<modules-remote-clusters, remote clusters>>.
 
 ==== Configuring replication
 
@@ -213,4 +214,4 @@ following process starts again.
 
 You can terminate replication with the
 {ref}/ccr-post-unfollow.html[unfollow API]. This API converts a follower index
-to a regular (non-follower) index.
+to a regular (non-follower) index.