Add Cloudron package to docs

[gramakri]

Hi Gitea team!
Thanks for Gitea. We have been maintaining a Gitea package for Cloudron for a couple of years now. The initial package was contributed to us by Dennis Schwerdel.

The source for the package is at https://git.cloudron.io/cloudron/gitea-app. We have browser tests for gitea as well in the tests directory. It will also test push/pull across gitea updates as well! You can give it a try in our demo instance at https://my-demo.cloudron.me (username: cloudron password: cloudron).

Would be great if you could add it as one of your installation methods.