Nextcloud Talk: fix TURN/STUN config

- Removed duplicate Admin Manual link in README
- Fixed turnserver.config.php draft return syntax
- Unified onboard port handling in docker-compose and env
- Updated vars to define STUN/TURN configs with correct schemas
- Ensured spreed plugin config serializes clean JSON arrays

Ref: https://chatgpt.com/share/68d7cfa2-7378-800f-9ecf-09b6bb768f13

roles/web-app-nextcloud/templates/config/turnserver.config.php.j2.draft

@@ -2,11 +2,13 @@
 # Activates the turn server
 # @see https://nextcloud-talk.readthedocs.io/en/latest/TURN/
 
-return 'turn_servers' => [
+return [
+  'turn_servers' => [
     [
         'host' => 'coturn',
-        'port' => {{ NEXTCLOUD_TALK_TURN_ONBOARD_STUN_PORT }},
+        'port' => {{ NEXTCLOUD_TALK_TURN_ONBOARD_PORT }},
         'secret' => 'my-secret-key',
         'protocols' => 'udp,tcp'
-    ]
+    ],
+  ],
 ];