Allow specifying the document structure in automerge-py

But it doesn't, it's still an automerge document and we need it to be. I think @rroohhh was working on better typing for the automerge-library that would allow us to give the proper typehints for this

_Originally posted by @pajowu in https://github.com/transcribee/transcribee/pull/118#discussion_r1183468456_