Centralize all documentations on Gitbook

Using Gitbook, we can still benefit from all the advantages of using git (Github WYSIWYG editor, versions) but also have some benefits:
- Nice UI
- Multilingual
- User feedback (good/neutral/bad)

By embedding it in Frontend with an iframe, we'll unify all documentations in the same place and we'll be able to update it without re-deploying.

# Steps

- [ ] Create Gitbook with a custom book.json
- [ ] Move all help page to repo `/docs`
- [ ] Add pages from Github Wiki
- [ ] Add multilanguage
- [ ] Embed on Frontend
- [ ] Bonus: Keep compatibility with old routes
- [ ] Update all existing routes
- [ ] Deprecate Github Wiki