[DOCS] Adds placeholders for v8 highlights, breaking changes, release notes (#38641)

Author: lcawl

docs/reference/migration/index.asciidoc

@@ -21,5 +21,7 @@ For more information, see <<setup-upgrade>>.
 
 See also <<release-highlights>> and <<es-release-notes>>.
 
+* <<breaking-changes-8.0,Breaking changes in 8.0>>
+
 --
-include::migrate_7_0.asciidoc[]
+include::migrate_8_0.asciidoc[]