[WIP] Move documentation to be generated by couscous

- [x] Move existent pages to be generated using couscous
- [x] Create extras pages in `md` to be converted to markdown
- [x] Script to generate documentation
- [x] Link documentation versions

These links should stay as `.md`. Couscous [seems to have pluggable link replacement steps](https://github.com/CouscousPHP/Couscous/blob/f5d798c60d05d33413665c0318608510eedc576d/src/Module/Markdown/Step/ProcessMarkdownLinks.php#L19)