Add documentation in OpenAPI format using adonis5-swagger

[Frewacom]

The OpenAPI format is widely accepted as the standard documentation method for API's and have a wide selection of front end generators, e.g. Redocly.

https://github.com/reg2005/adonis5-swagger

I have looked at a few different front end generators, and Redocly is the one that seems the best, both in design and functionality.

Tasks

• Install and setup adonis5-swagger
• Document all of the endpoints 🙉
• Install Redocly and serve the static files at e.g. <domain>/docs (from the public directory)
  Another solution would be to deploy the docs separately like we are currently doing, but this will require more work to make it fully automated.
• Automate the docs generation in staging and production so that it is automatically available after deploying.