Docs for Gitea Actions (#24405)

Docs for Gitea Actions (#24405)

A new documentation section for Gitea Actions.

Some content comes from:

- [Hacking on Gitea
Actions](https://blog.gitea.io/2023/03/hacking-on-gitea-actions/)
- The README of [act_runner](https://gitea.com/gitea/act_runner)
- @ChristopherHX's excellent overview of the differences between Gitea
Actions and GitHub Actions in [this
comment](https://github.com/go-gitea/gitea/issues/13539#issuecomment-1448888850).

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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>