docs: update moderated sessions guide

Moderated sessions are a bit confusing because we combined an existing
OSS feature (joining sessions), with an Enterprise-only feature
(requiring session join policies).

I've expanded the scope of the moderated sessions guide to make it a
"joining sessions" guide instead, and added some extra details around
RBAC for active sessions.

Updates #51116

CHANGELOG.md

@@ -5468,7 +5468,7 @@ sessions remains deny-by-default but now only `join_sessions` statements are
 checked for session join RBAC.
 
 See the [Moderated Sessions
-guide](docs/pages/admin-guides/access-controls/guides/moderated-sessions.mdx) for more
+guide](docs/pages/admin-guides/access-controls/guides/joining-sessions.mdx) for more
 details.
 
 #### GitHub connectors
@@ -5927,7 +5927,7 @@ With Moderated Sessions, Teleport administrators can define policies that allow
 users to invite other users to participate in SSH or Kubernetes sessions as
 observers, moderators or peers.
 
-[Moderated Sessions guide](docs/pages/admin-guides/access-controls/guides/moderated-sessions.mdx)
+[Moderated Sessions guide](docs/pages/admin-guides/access-controls/guides/joining-sessions.mdx)
 
 ### Breaking Changes
 

docs/config.json

@@ -218,6 +218,11 @@
       "destination": "/enroll-resources/application-access/introduction/",
       "permanent": true
     },
+    {
+      "source": "/admin-guides/access-controls/guides/moderated-sessions/",
+      "destination": "/admin-guides/access-controls/guides/joining-sessions/",
+      "permanent": true
+    },
     {
       "source": "/setup/admin/github-sso/",
       "destination": "/admin-guides/access-controls/sso/github-sso/",
@@ -1645,7 +1650,7 @@
     },
     {
       "source": "/access-controls/guides/moderated-sessions/",
-      "destination": "/admin-guides/access-controls/guides/moderated-sessions/",
+      "destination": "/admin-guides/access-controls/guides/joining-sessions/",
       "permanent": true
     },
     {