Add service definition for Forgejo and Gitea

Description: Gitea and Forgejo are Open Source, Self-Hostable DevOps Platforms.

Official Website: https://about.gitea.com/ | https://forgejo.org/
Demo Website: https://demo.gitea.com/ | https://v13.next.forgejo.org/

Default Ports: 3000

Discovery Method

• Pattern type: Endpoint response body from :3000/explore/repos contains "Gitea" (or "Forgejo" as applicable)
• Reasoning: Gitea uses http/3000 as it's listening port, and every page includes "Powered by Gitea" in the footer. The path / isn't used as some instances disable the landing page and redirect to explore directly. See my instance at https://git.konpeki.solutions for an example of this.

Icon Source: Dashboard Icons

Testing:

• Compiles successfully
• Tested against real instance (describe setup below)
• Unable to test (explain why below)

Testing Details:

• Forgejo Tested by running a daemon locally and pulling a docker image with docker run -p 3000:3000 codeberg.org/forgejo/forgejo:13. Navigate to http://localhost:3000, allow self registration, and finish the install. Then prompt network discovery.
  Gitea testing is the exact same, using docker.gitea.com/gitea:latest instead of codeberg.org/forgejo/forgejo:13

Additional Notes:

• Contributing Guidelines state "One service definition per PR", however as Forgejo is originally a fork of Gitea and their detections are nearly identical I'm bundling them together.
• /explore/repo is a bit verbose to meet the case mentioned above. / is possible, though may miss some non-default instances
• Powered by Gitea / Powered by Forgejo is particularly verbose, especially when used in conjunction with /explore/repo. This can be cut down to Gitea and Forgejo if that is preferred.