Make OpenAPI docs from RestXQ annotations

[dvglc]

For instance, using https://lab.sub.uni-goettingen.de/openapi4restxq.html (I haven't managed to get this to work reasonably fast, though...). There might be other tools available as well.

Once available, the API default handler could redirect to the OpenAPI docs in case of an underspecified request (instead of throwing an 400 error, as is the case ATM).

[dvglc]

Perhaps the easiest way for generating OpenAPI docs would be a self-written program for transforming the output of a restxq registry function (such as exrest:register-module()) into an OpenAPI-json format. See the output of $config:app-root/register-rest.xql, for example.