[DOCS] Added settings page for ILM. (#40880)

* [DOCS] Added settings page for ILM.

* [DOCS] Adding ILM settings file

* [DOCS] Moved the ILM settings to a separate section

* [DOCS] Linked to the rollover docs.

* [DOCS] Tweaked the "required" wording.

docs/reference/ilm/getting-started-ilm.asciidoc

@@ -15,7 +15,9 @@ our writing index. We wish to roll over the index after it reaches a size
 of 50 gigabytes, or has been created 30 days ago, and then delete the index
 after 90 days.
 
-=== Setting up a new policy
+[float]
+[[ilm-gs-create-policy]]
+=== Setting up a policy
 
 There are many new features introduced by {ilm-init}, but we will only focus on
 a few that are needed for our example. For starters, we will use the
@@ -64,6 +66,8 @@ the index being written to after it reaches 50 gigabytes, or it is 30
 days old. The rollover will occur when either of these conditions is true.
 The index will be deleted 90 days after it is rolled over.
 
+[float]
+[[ilm-gs-apply-policy]]
 === Applying a policy to our index
 
 There are <<set-up-lifecycle-policy,a few ways>> to associate a
@@ -135,6 +139,8 @@ index being the index that is being written to at a time. Rollover swaps
 the write index to be the new index created from rollover, and sets the
 alias to be read-only for the source index.
 
+[float]
+[[ilm-gs-check-progress]]
 === Checking progress
 
 Now that we have an index managed by our policy, how do we tell what is going

docs/reference/index-modules.asciidoc

@@ -285,6 +285,12 @@ Other index settings are available in index modules:
 
     Control over the transaction log and background flush operations.
 
+[float]
+=== [xpack]#{xpack} index settings#
+
+<<ilm-settings,{ilm-cap}>>::
+
+    Specify the lifecycle policy and rollover alias for an index.
 --
 
 include::index-modules/analysis.asciidoc[]

docs/reference/settings/ilm-settings.asciidoc

@@ -0,0 +1,15 @@
+[role="xpack"]
+[[ilm-settings]]
+=== {ilm-cap} settings
+
+These index-level {ilm-init} settings are typically configured through index
+templates. For more information, see <<ilm-gs-create-policy>>.
+
+`index.lifecycle.name`::
+The name of the policy to use to manage the index.
+
+`index.lifecycle.rollover_alias`::
+The index alias to update when the index rolls over. Specify when using a
+policy that contains a rollover action. When the index rolls over, the alias is
+updated to reflect that the index is no longer the write index. For more
+information about rollover, see <<using-policies-rollover>>.

docs/reference/setup.asciidoc

@@ -52,6 +52,8 @@ include::settings/audit-settings.asciidoc[]
 
 include::settings/ccr-settings.asciidoc[]
 
+include::settings/ilm-settings.asciidoc[]
+
 include::settings/license-settings.asciidoc[]
 
 include::settings/ml-settings.asciidoc[]