Add docs for ILM feature (elastic#9263) (elastic#10264)

* Add docs for ILM feature

This is a follow up for elastic#7963

* Add edits

* Minor fixes

* Add changes from the review

* Make cross references consistent

* Fix rebase error

docs/configuring-howto.asciidoc

@@ -16,6 +16,7 @@ The following topics describe how to configure {beatname_uc}:
 * <<configuration-general-options>>
 * <<configuring-internal-queue>>
 * <<configuring-output>>
+* <<ilm>>
 * <<configuration-ssl>>
 * <<filtering-and-enhancing-data>>
 * <<configuring-ingest-node>>
@@ -39,6 +40,8 @@ include::{libbeat-dir}/docs/queueconfig.asciidoc[]
 
 include::{libbeat-dir}/docs/outputconfig.asciidoc[]
 
+include::{libbeat-dir}/docs/shared-ilm.asciidoc[]
+
 include::{libbeat-dir}/docs/shared-ssl-config.asciidoc[]
 
 include::./filtering.asciidoc[]