Adding [Gitpod](https://github.com/gitpod-io/gitpod) allows us and others to easily start a complete Woodpecker setup and development cloud IDE. It starts a Woodpecker server, agent and a preconfigured Gitea instance. You can login at Gitea with `woodpecker` and `password`.
- migrate step conditions back into pipeline syntax, but show 2-4 level in toc to be able to see `when` keywords
- create new backend section in admin docs
- update docusaurus
- remove prefix docker of container / container-image where possible
- replace terms SCM, VCS, Github with [forge](https://en.wikipedia.org/wiki/Forge_(software))
- add darkmode favicon variant
- Replace mkdocs with docosaurus (improved menu structure, ...)
- Structure docs into `Usage` and `Server Setup / Administration`
- Update favicon
- Create new pipeline-syntax page with all options and links to more detailed docs if available
- Add ci to publish to `woodpecker-ci.github.io`
- Deploy docs preview to surge for review
- Update start-page
Co-authored-by: 6543 <6543@obermui.de>
