Use openapi4restxq to generate API documentation

[cmil]

Instead of maintaining api.yml manually we should let the API generate its own documentation from xqDoc using the openapi4restxq package.

See also https://lab.sub.uni-goettingen.de/openapi4restxq.html.

Builds:

• from master: https://ci.de.dariah.eu/exist-repo/packages.html?package-id=openapi&eXist-db-min-version=4.6.0
• from develop: https://ci.de.dariah.eu/exist-repo/packages.html?package-id=openapi-develop&eXist-db-min-version=4.6.0