[DOCS] Add CCR docs to the Elasticsearch Reference Guide (#45704)

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.

docs/reference/administering.asciidoc

@@ -1,12 +0,0 @@
-[[administer-elasticsearch]]
-= Administering {es}
-
-[partintro]
---
-Elasticsearch is a complex piece of software, with many moving parts. There are
-many APIs and features that are designed to help you manage your Elasticsearch
-cluster.
-
---
-
-include::administering/backup-cluster.asciidoc[]

docs/reference/ccr/auto-follow.asciidoc

@@ -1,7 +1,7 @@
 [role="xpack"]
 [testenv="platinum"]
 [[ccr-auto-follow]]
-=== Automatically following indices
+== Automatically following indices
 
 In time series use cases where you want to follow new indices that are
 periodically created (such as daily Beats indices), manually configuring follower
@@ -10,7 +10,7 @@ functionality in {ccr} is aimed at easing this burden. With the auto-follow
 functionality, you can specify that new indices in a remote cluster that have a 
 name that matches a pattern are automatically followed.
 
-==== Managing auto-follow patterns
+=== Managing auto-follow patterns
 
 You can add a new auto-follow pattern configuration with the
 {ref}/ccr-put-auto-follow-pattern.html[create auto-follow pattern API].  When you create

docs/reference/ccr/getting-started.asciidoc

@@ -20,7 +20,7 @@ This getting-started guide for {ccr} shows you how to:
 
 . Obtain a license that includes the {ccr} features. See
   https://www.elastic.co/subscriptions[subscriptions] and
-  <<license-management>>.
+  {stack-ov}/license-management.html[License-management].
 
 . If the Elastic {security-features} are enabled in your local and remote
   clusters, you need a user that has appropriate authority to perform the steps
@@ -34,7 +34,7 @@ to control which users have authority to manage {ccr}.
 By default, you can perform all of the steps in this tutorial by
 using the built-in `elastic` user. However, a password must be set for this user
 before the user can do anything. For information about how to set that password,
-see <<security-getting-started>>.
+see {stack-ov}/security-getting-started.html[Tutorial: Getting started with security].
 
 If you are performing these steps in a production environment, take extra care
 because the `elastic` user has the `superuser` role and you could inadvertently

docs/reference/ccr/index.asciidoc

@@ -3,9 +3,6 @@
 [[xpack-ccr]]
 = {ccr-cap}
 
-[partintro]
---
-
 The {ccr} (CCR) feature enables replication of indices in remote clusters to a
 local cluster. This functionality can be used in some common production use
 cases:
@@ -22,7 +19,6 @@ This guide provides an overview of {ccr}:
 * <<ccr-getting-started>>
 * <<ccr-upgrading>>
 
---
 
 include::overview.asciidoc[]
 include::requirements.asciidoc[]

docs/reference/ccr/requirements.asciidoc

@@ -1,7 +1,7 @@
 [role="xpack"]
 [testenv="platinum"]
 [[ccr-requirements]]
-=== Requirements for leader indices
+== Requirements for leader indices
 
 {ccr-cap} works by replaying the history of individual write
 operations that were performed on the shards of the leader index. This means that the
@@ -24,7 +24,7 @@ enabled.
 
 [float]
 [[ccr-overview-soft-deletes]]
-==== Soft delete settings
+=== Soft delete settings
 
 `index.soft_deletes.enabled`::
 
@@ -44,7 +44,7 @@ For more information about index settings, see {ref}/index-modules.html[Index mo
 
 [float]
 [[ccr-overview-beats]]
-==== Setting soft deletes on indices created by APM Server or Beats
+=== Setting soft deletes on indices created by APM Server or Beats
 
 If you want to replicate indices created by APM Server or Beats, and are
 allowing APM Server or Beats to manage index templates, you need to configure
@@ -65,7 +65,7 @@ index template.
 
 [float]
 [[ccr-overview-logstash]]
-==== Setting soft deletes on indices created by Logstash
+=== Setting soft deletes on indices created by Logstash
 
 If you want to replicate indices created by Logstash, and are using Logstash to
 manage index templates, you need to configure soft deletes on a custom Logstash

docs/reference/high-availability.asciidoc

@@ -0,0 +1,35 @@
+[[high-availability]]
+= Set up a cluster for high availability
+
+[partintro]
+--
+As with any software that stores data,
+it is important to routinely back up your data.
+{es}'s <<glossary-replica-shard,replica shards>> provide high availability
+during runtime;
+they enable you to tolerate sporadic node loss
+without an interruption of service.
+
+However, replica shards do not protect an {es} cluster
+from catastrophic failure.
+You need a backup of your cluster—
+a copy in case something goes wrong.
+
+
+{es} offers two features to support high availability for a cluster:
+
+* <<backup-cluster,Snapshot and restore>>,
+which you can use to back up individual indices or entire clusters.
+You can automatically store these backups in a repository on a shared filesystem.
+
+* <<xpack-ccr,Cross-cluster replication (CCR)>>,
+which you can use to copy indices in remote clusters to a local cluster.
+You can use {ccr} to recover from the failure of a primary cluster
+or serve data locally based on geo-proximity.
+--
+
+include::high-availability/backup-cluster.asciidoc[]
+
+:leveloffset: +1
+include::ccr/index.asciidoc[]
+:leveloffset: -1

docs/reference/high-availability/backup-cluster-data.asciidoc

@@ -4,14 +4,6 @@
 <titleabbrev>Back up the data</titleabbrev>
 ++++
 
-As with any software that stores data, it is important to routinely back up your
-data. {es} replicas provide high availability during runtime; they enable you to
-tolerate sporadic node loss without an interruption of service.
-
-Replicas do not provide protection from catastrophic failure, however. For that,
-you need a real backup of your cluster—a complete copy in case something goes
-wrong.
-
 To back up your cluster's data, you can use the <<modules-snapshots,snapshot API>>.
 
 include::{es-repo-dir}/modules/snapshots.asciidoc[tag=snapshot-intro]

docs/reference/index.asciidoc

@@ -56,7 +56,7 @@ include::rollup/index.asciidoc[]
 
 include::frozen-indices.asciidoc[]
 
-include::administering.asciidoc[]
+include::high-availability.asciidoc[]
 
 include::commands/index.asciidoc[]
 

docs/reference/redirects.asciidoc

@@ -815,4 +815,8 @@ See <<getting-started-next-steps>>.
 
 [role="exclude",id="ccs-reduction"]
 === {ccs-cap} reduction
-See <<ccs-works>>.
+See <<ccs-works>>.
+
+[role="exclude",id="administer-elasticsearch"]
+=== Administering {es}
+See <<high-availability>>.