add prepare_documents

[ArneBinder]

Simple approach to handle #101. This also allows the user to easily integrate sophisticated logic to e.g. create candidate entity pairs for RE.

[ChristophAlt]

Why add another method? This only makes the workflow more complex to understand for users. How about adding a is_training: bool = False parameter to encode_inputand set it to encode_target in encode(...)?

[ChristophAlt]

And while looking at the taskmodule I'd also recommend to remove the combine_output and create_annotations_from_output methods in a later commit, as it unnecessarily restricts the user and makes the workflow more complicated to understand and implement.