Add cert info to third_party nextcloud doc

Add instructions around configuring ADDITIONAL_SAN for the Nextcloud domain with a reference to more detailed documentation.

docs/third_party/third_party-nextcloud.en.md

@@ -7,6 +7,9 @@ Nextcloud can be set up (parameter `-i`) and removed (parameter `-p`) with the [
 
 In case you have forgotten the password (e.g. for admin) and can't request a new one [via the password reset link on the login screen](https://docs.nextcloud.com/server/20/admin_manual/configuration_user/reset_admin_password.html?highlight=reset) calling the helper script with `-r` as parameter allows you to set a new password. Only use this option if your Nextcloud isn't configured to use mailcow for authentication as described in the next section.
 
+In order for mailcow to generate a a certificate for the nextcloud domain you need to add "nextcloud.domain.tld" to ADDITIONAL_SAN in mailcow.conf and run `docker-compose up -d` to apply. For more informaton refer to: [Advanced SSL](https://mailcow.github.io/mailcow-dockerized-docs/post_installation/firststeps-ssl/).
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 ## Background jobs
 
 To use the recommended setting (cron) to execute the background jobs following lines need to be added to the `docker-compose.override.yml`: