2.2.7 Publish the API documentation online

At the very least, a link to the versioned Swagger file must be added to the README.

It will help front-end developers understand the API endpoints, and serve as a basis for future work on new Garden Party clients.

Such link should be located at the top of the README.

As we now have a documentation system for VueJS components and other pages, I think we should integrate the API documentation in it, and publish it on Gitlab pages.

This documentation, targetting developpers and contributors may be accessible on a sub-domain as docs.garden-party.io or something like that, and be referenced in the README.