[DOCS] Remove unused upgrade doc files (#83617)

* Removes several unused asciidoc files for upgrade docs
* Fixes a redirect for "Rolling upgrade"
* Adds redirects for "Reindex before upgrading," "Reindex in place," and "Reindex from a remote cluster"

Relates to #83489 and
elastic/docs#2312

Author: jrodewig

docs/reference/redirects.asciidoc

@@ -1808,13 +1808,42 @@ See the <<sql-search-api-request-body,request body parameters>> for the
 
 When upgrading to {es} 8.0 and later, you must first upgrade to {prev-major-last}
 even if you opt to perform a full-cluster restart instead of a rolling upgrade.
-For more information about upgrading, see 
+For more information about upgrading, refer to 
 {stack-ref}/upgrading-elastic-stack.html[Upgrading to Elastic {version}].
 
-role="exclude",id="rolling-upgrade"]
+[role="exclude",id="rolling-upgrades"]
 === Rolling upgrade
 
 When upgrading to {es} 8.0 and later, you must first upgrade to {prev-major-last}
 whether you opt to perform a rolling upgrade (upgrade one node at a time) or a full-cluster restart upgrade.
-For more information about upgrading, see 
+For more information about upgrading, refer to 
 {stack-ref}/upgrading-elastic-stack.html[Upgrading to Elastic {version}].
+
+[role="exclude",id="reindex-upgrade"]
+=== Reindex before upgrading
+
+//tag::upgrade-reindex[]
+Before upgrading to {es} 8.0 and later, you must reindex any indices created in
+a 6.x version. We recommend using the **Upgrade Assistant** to guide you
+through this process.
+
+For more information about upgrading, refer to 
+{stack-ref}/upgrading-elastic-stack.html[Upgrading to Elastic {version}].
+//end::upgrade-reindex[]
+
+For more information about reindexing, refer to <<docs-reindex>>.
+
+[role="exclude",id="reindex-upgrade-inplace"]
+=== Reindex in place
+
+include::redirects.asciidoc[tag=upgrade-reindex]
+
+For more information about reindexing, refer to <<docs-reindex>>.
+
+[role="exclude",id="reindex-upgrade-remote"]
+=== Reindex from a remote cluster
+
+include::redirects.asciidoc[tag=upgrade-reindex]
+
+For more information about reindexing from a remote cluster, refer to
+<<reindex-from-remote>>.