[migration] Remove AsciiDoc files (#1805)

* delete files from fleet and ea guide

* delete files from ingest overview guide

* update readme

Author: colleenmcginnis

README.md

@@ -1,9 +1,17 @@
+> [!IMPORTANT]
+> Starting with Elastic Stack v9.0.0 the content in this repo has moved.
+>
+> **If you need to update the docs for a version prior to 9.0.0**, you can open a PR in this repo targeting the specific version branch and backport to other branches using the relevant `backport-{branch}` labels.
+>
+> **If you're updating the docs for version 9.0 or later**, you can open a PR in the content's new home: [elastic/docs-content](https://github.com/elastic/docs-content).
+
 # ingest-docs
+
 Home for Elastic ingest documentation
 
 ## Backporting
 
-Pull requests should be tagged with the target version of the Elastic Stack along with any relevant backport labels. In general, we only backport documentation changes to [live stack versions](https://github.com/elastic/docs/blob/master/conf.yaml#L80). For manual backports, we recommend using the [backport tool](https://github.com/sqren/backport) to easily open backport PRs. If you need help, ping **[ingest-docs](https://github.com/orgs/elastic/teams/ingest-docs)** and we'd be happy to handle the backport process for you.
+Pull requests should be tagged with the target version of the Elastic Stack along with any relevant backport labels. In general, we only backport documentation changes to [live stack versions](https://github.com/elastic/docs/blob/master/conf.yaml#L80). If you need help, ping **[ingest-docs](https://github.com/orgs/elastic/teams/ingest-docs)** and we'd be happy to handle the backport process for you.
 
 ## License