Provide a contribution guide

[DanielSiepmann]

Not everyone might now how we work and how GitHub works.
We therefore should provide a contribution guide. GitHub provides a link to a guide if we follow the docs: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

The guide should probably explain how to contribute in general. https://docs.typo3.org/Home/Contribute.html does already exist, but contribution to docs and TYPO3 core is different from contributing to TYPO3 extensions on GitHub and GitLab. This workflow should be covered. E.g. First create a fork, followed by a new branch. Apply your changes and create a pull request.

This should lower the barrier for others to contribute. Especially those who are not using GitHub on a daily basis.

[DanielSiepmann]

Done with #1044, available at: https://github.com/FriendsOfTYPO3/tea/blob/main/CONTRIBUTING.md