Specifies Elastic 9.0 and Elastic Cloud Serverless on main release notes (#1117)

Clarifies that latest release notes include Elastic Stack 9.0 and
Elastic Cloud Serverless.

Adds link to release notes for previous version.

Author: KOTungseth

release-notes/intro/index.md

@@ -6,7 +6,9 @@ mapped_pages:
 
 # Release notes [elastic-release-notes]
 
-Learn about the latest changes, issues, fixes, and deprecations for Elastic releases by reviewing the release notes.
+Learn about the latest changes, issues, fixes, and deprecations for Elastic releases, including {{stack}} 9.0 and {{serverless-full}}, by reviewing the release notes.
+
+To view the changes in previous versions, go to [Release docs](https://www.elastic.co/guide/en/starting-with-the-elasticsearch-platform-and-its-solutions/8.x/new.html).
 
 For information about the latest changes to Elastic APIs, check [APIs changelog](https://www.elastic.co/docs/api/changes).