Document how merges to main deploy through Dokploy's GitHub App

DEPLOYMENT.md covers the image, migrations and runtime settings, but it doesn't say how a merge to main reaches Dokploy. Merges weren't deploying for two reasons:

• Dokploy set its GitHub App's webhook URL from the private address the dashboard was open at, which GitHub can't reach.
• A repository webhook sent form-encoded payloads, and Dokploy answered every one with 301 "Branch Not Match".

Do

• Add a short section to DEPLOYMENT.md saying that:
  • the GitHub App webhook (/api/deploy/github) does the triggering, and it chooses which apps to deploy by provider, owner, repository, branch and trigger;
  • the App's Webhook URL has to be publicly reachable, and here is how to change it;
  • you shouldn't also add a repository webhook with the app's own URL, because every merge would then deploy twice;
  • if you do use that URL, the content type must be JSON.

This is a docs-only change. Merging it also serves as a test that auto-deploy now works.