[DOCS] Adds notable changes section (#38238)

Author: lcawl

docs/reference/migration/migrate_8_0.asciidoc

@@ -11,13 +11,20 @@ See also <<release-highlights>> and <<es-release-notes>>.
 
 coming[8.0.0]
 
+* <<breaking_80_notable>>
 * <<breaking_80_analysis_changes>>
 * <<breaking_80_discovery_changes>>
 * <<breaking_80_mappings_changes>>
 * <<breaking_80_snapshots_changes>>
 
 [float]
-=== Indices created before 7.0
+[[breaking_80_notable]]
+=== Notable changes
+// NOTE: The content in this section is also used in the Installation and Upgrade Guide.
+
+//tag::notable-breaking-changes[]
+[float]
+==== Indices created before 7.0
 
 Elasticsearch 8.0 can read indices created in version 7.0 or above.  An
 Elasticsearch 8.0 node will not start in the presence of indices created in a
@@ -31,6 +38,7 @@ Indices created in Elasticsearch 6.x or before will need to be reindexed with
 Elasticsearch 7.x in order to be readable by Elasticsearch 8.x.
 
 =========================================
+// end::notable-breaking-changes[]
 
 include::migrate_8_0/analysis.asciidoc[]
 include::migrate_8_0/discovery.asciidoc[]