[DOCS] Added link to upgrade guide and bumped the upgrade topic up to the top level (#27621)

* [DOCS] Added link to the upgrade guide & tweaked the intro.

* [DOCS] Bumped upgrade topic up to the top level of the TOC

docs/reference/index-shared1.asciidoc

@@ -2,3 +2,5 @@
 include::getting-started.asciidoc[]
 
 include::setup.asciidoc[]
+
+include::upgrade.asciidoc[]

docs/reference/setup.asciidoc

@@ -50,6 +50,4 @@ include::setup/sysconfig.asciidoc[]
 
 include::setup/bootstrap-checks.asciidoc[]
 
-include::setup/upgrade.asciidoc[]
-
 include::setup/stopping.asciidoc[]

docs/reference/upgrade.asciidoc

@@ -1,5 +1,19 @@
 [[setup-upgrade]]
-== Upgrading Elasticsearch
+= Upgrade Elasticsearch
+
+[partintro]
+--
+Elasticsearch can usually be upgraded using a <<rolling-upgrades,Rolling upgrade>>
+process so upgrading does not interrupt service. However, you might
+need to <<reindex-upgrade,Reindex to upgrade>> indices created in older versions.
+Upgrades across major versions prior to 6.0 require a <<restart-upgrade,Full cluster restart>>.
+
+When upgrading to a new version of Elasticsearch, you need to upgrade each
+of the products in your Elastic Stack. The steps you need to take to upgrade
+differ depending on which products you are using. Want a list that's tailored
+to your stack? Try out our {upgrade_guide}[Interactive Upgrade Guide]. For
+more information about upgrading your stack, see {stack-ref}[Upgrading the
+Elastic Stack].
 
 [IMPORTANT]
 ===========================================
@@ -17,10 +31,6 @@ your data.
 
 ===========================================
 
-Elasticsearch can usually be upgraded using a <<rolling-upgrades,Rolling upgrade>>
-process so upgrading does not interrupt service. However, you might
-need to <<reindex-upgrade,Reindex to upgrade>> indices created in older versions.
-Upgrades across major versions prior to 6.0 require a <<restart-upgrade,Full cluster restart>>.
 
 The following table shows when you can perform a rolling upgrade, when you
 need to reindex or delete old indices, and when a full cluster restart is
@@ -56,7 +66,7 @@ For information about how to upgrade old indices, see <<reindex-upgrade,
 Reindex to upgrade>>.
 
 ===============================================
-
+--
 
 include::upgrade/rolling_upgrade.asciidoc[]
 

docs/reference/upgrade/cluster_restart.asciidoc

@@ -1,5 +1,5 @@
 [[restart-upgrade]]
-=== Full cluster restart upgrade
+== Full cluster restart upgrade
 
 A full cluster restart upgrade requires that you shut all nodes in the cluster
 down, upgrade them, and restart the cluster. A full cluster restart was required

docs/reference/upgrade/reindex_upgrade.asciidoc

@@ -1,5 +1,5 @@
 [[reindex-upgrade]]
-=== Reindex before upgrading
+== Reindex before upgrading
 
 Elasticsearch can read indices created in the *previous major version*.
 Older indices must be reindexed or deleted. Elasticsearch 6.x
@@ -56,7 +56,7 @@ been deleted.
 
 
 [[reindex-upgrade-inplace]]
-==== Reindex in place
+=== Reindex in place
 
 To manually reindex your old indices with the <<docs-reindex,`reindex` API>>:
 
@@ -88,7 +88,7 @@ regular part of your Elastic Stack. For more information, see
 *******************************************
 
 [[reindex-upgrade-remote]]
-==== Reindex from a remote cluster
+=== Reindex from a remote cluster
 
 You can use <<reindex-from-remote,reindex from remote>> to migrate indices from
 your old cluster to a new 6.x cluster. This enables you move to 6.x from a

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -1,5 +1,5 @@
 [[rolling-upgrades]]
-=== Rolling upgrades
+== Rolling upgrades
 
 A rolling upgrade allows an Elasticsearch cluster to be upgraded one node at
 a time so upgrading does not interrupt service. Running multiple versions of