Document required Gitea webhook whitelist config

The Gitea installation process as currently described leads to non-working webhooks with the following error showing up in Gitea logs:

Delivery: Post "": dial tcp webhook can only call allowed HTTP servers (check your webhook.ALLOWED_HOST_LIST setting), deny ''

This issue and its solution has been described here before: Gitea Webhook Change

The Drone installation docs for Gitea should mention this.