Add a method to save a doc to conllu format

Currently the `repr(doc)` of a doc produces the concatenation of the CoNLL-U representation of all sentences but without the comments indicating medata like sentence ids.

A method to save to a CoNLL-U file with these metadata should be added.

Start by tracing the implementation of repr(doc) and the document and sentence representations it uses. Compare the existing CoNLL-U output with the metadata comments required by the issue, then identify the public entry point for saving a document. Done means a save method produces a CoNLL-U file containing the sentence metadata comments as well as the sentence representations.

Written by the indexing model from the issue text.