Extract documentation to Gradle subproject

This is some experiment I did in order to decrease build time. It should lead to less cache validations and does improve things for me when building the whole project, but still runs :rest-api-server:integrationTest too often, because the tests do not produce stable output.

This is partially also for serving the documentation directly from the server, so that it always matches the version running. (This does not work yet.) It is (kind of) a requirement when we want to support OpenAPI, and I would like to base that branch on this one.

I am happy for feedback, and will also improve some things next week, most likely. In its current state it should be working, although it does not improve a lot, unfortunately.