Explain how to scan and fix Nextcloud instance

Guides/nextcloud.md

@@ -57,3 +57,7 @@ No need to restart system, config is taken into account at web page reload.
 Follow [official instructions on Nextcloud doc](https://docs.nextcloud.com/server/latest/admin_manual/installation/harden_server.html)
 
 To enable *mod_headers* on HTTPD : `a2enmod headers`
+
+## Other fixes
+
+As an admin, go to _Overview_ in the _Administration_ panel from _Settings_. Nexctloud will scan your instance and give you instructions.