Add more docs

[flowirtz]

This PR adds a lot more documentation content, as well as making some minor cosmetic adjustments.

OpenAPI

Also adding an example OpenAPI spec (petstore), via here.

[dlicheva]

This looks good generally, but I think we shouldn't add such a large mock documentation website to the repo. Maybe instead of the petstore here, we can add the actual structure of our own documentation in a separate PR.

[dlicheva]

Nice! 🙌