Dropped support for `DRONE_*` environment variables in pipeline steps. Pipeline meta-data can be accessed with `CI_*` variables.
- `CI_*` prefix replaces `DRONE_*`
- `CI` value is now `woodpecker`
- `DRONE=true` has been removed
- 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>