[Upgrade 9.0] Revises TOC structure for "Upgrade" section (#823)

Restructures the Upgrade TOC. 


[Preview](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/823/deploy-manage).

Author: jmikell821

deploy-manage/toc.yml

@@ -793,16 +793,17 @@ toc:
       - file: maintenance/add-and-remove-elasticsearch-nodes.md
   - file: upgrade.md
     children:
-      - file: upgrade/prepare-to-upgrade.md
-        children:
-          - file: upgrade/prepare-to-upgrade/upgrade-assistant.md
-          - file: upgrade/prepare-to-upgrade/index-compatibility.md
+      - file: upgrade/plan-upgrade.md
       - file: upgrade/orchestrator.md
         children:
           - file: upgrade/orchestrator/upgrade-cloud-enterprise.md
             children:
               - file: upgrade/orchestrator/re-running-the-ece-upgrade.md
           - file: upgrade/orchestrator/upgrade-cloud-on-k8s.md
+      - file: upgrade/prepare-to-upgrade.md
+        children:
+          - file: upgrade/prepare-to-upgrade/upgrade-assistant.md
+          - file: upgrade/prepare-to-upgrade/index-compatibility.md
       - file: upgrade/deployment-or-cluster.md
         children:
           - file: upgrade/deployment-or-cluster/upgrade-on-ech.md
@@ -811,12 +812,12 @@ toc:
           - file: upgrade/deployment-or-cluster/self-managed.md
             children:
               - file: upgrade/deployment-or-cluster/elasticsearch.md
+                children:
+                  - file: upgrade/deployment-or-cluster/archived-settings.md
+                  - file: upgrade/deployment-or-cluster/reading-indices-from-older-elasticsearch-versions.md
               - file: upgrade/deployment-or-cluster/kibana.md
                 children:
-                  - file: upgrade/deployment-or-cluster/kibana-roll-back.md
-          - file: upgrade/deployment-or-cluster/archived-settings.md
-            children:
-              - file: upgrade/deployment-or-cluster/reading-indices-from-older-elasticsearch-versions.md
+                  - file: upgrade/deployment-or-cluster/kibana-roll-back.md    
       - file: upgrade/ingest-components.md
       - file: upgrade/internal-upgrade-processes.md
         children:

deploy-manage/upgrade/deployment-or-cluster/self-managed.md

@@ -1,4 +1,4 @@
-# Upgrade Elastic on self-managed infrastructure
+# Upgrade Elastic on a self-managed cluster
 
 If you've installed the {{stack}} on your own self-managed infrastructure, once you're [prepared to upgrade](/deploy-manage/upgrade/deployment-or-cluster.md#prepare-to-upgrade), you'll need to upgrade each of your Elastic components individually. 
 

deploy-manage/upgrade/plan-upgrade.md

@@ -0,0 +1 @@
+# Plan your upgrade