Remove documents endpoint

[GeorgDangl]

We just discussed this very briefly in the group call.

The documents endpoint isn't used a lot, and there's now a dedicated API with the OpenCDE Documents API for it. So it might make sense to drop support for documents here in the BCF API, since it was always only intended as a small feature that allows to upload document files.

Should also remove document_references then of course😀

[GeorgDangl]

Once we discuss that, we might have to think more about document_references, if they should be kept or removed.

[GeorgDangl]

In the DocumentsAPI, we're able to create url references with a custom scheme: https://github.com/buildingSMART/documents-API#2231-bcf-file-references

So we can drop the documents endpoint here, but keep the document_references available and limit them to DocumentsAPI integration.

From Jon Anders: This is currently the only non-interactive way to upload documents.

@jasollien @pasi-paasiala Could you two provide a bit of feedback (short) about how the documents endpoint is currently used?

[GeorgDangl]

After a quick discussion, it seems that we're not really using these endpoints anywhere, so they can be removed.