Add the Upgrading Reference to the sidebar

Backports #46490

Closes #46124

Add the Upgrading Reference as a sidebar entry to `config.json`, giving
it a more explicit slug.

docs/config.json

@@ -31,7 +31,11 @@
             {
               "title": "Set up Automatic Agent Updates",
               "slug": "/upgrading/automatic-agent-updates/"
-            }
+            },
+            {
+              "title": "Upgrading Reference",
+              "slug": "/upgrading/upgrading-reference/"
+	    }
           ]
         },
         {

docs/pages/upgrading.mdx

@@ -17,7 +17,7 @@ Teleport cluster. You can also set up automatic agent upgrades in a self-hosted
 Enterprise cluster.
 
 For more information about upgrading, for example, to upgrade manually, read the
-[Upgrading Reference](upgrading/upgrading.mdx).
+[Upgrading Reference](upgrading/upgrading-reference.mdx).
 
 You can find more information regarding the automatic updates architecture in the
 [Agent Update Management](reference/architecture/agent-update-management.mdx) page.

docs/pages/upgrading/automatic-agent-updates.mdx

@@ -35,7 +35,7 @@ regular maintenance window.
 
 - A Teleport Enterprise cluster. If you do not have one, [sign
   up](https://goteleport.com/signup) for a free trial or consult the [Update
-  Reference](upgrading.mdx) to read about manual updates.
+  Reference](upgrading-reference.mdx) to read about manual updates.
 - Familiarity with the [Upgrading Compatibility Overview](./overview.mdx) guide,
   which describes the sequence in which to upgrade components in your cluster.
 - Teleport agents that are not yet enrolled in automatic updates.