📝 Show how to get v1-style entry dicts in migration guide

[MiWeiss]

The migration guide did not show how to get the plain list-of-dicts / {key: dict} that v1's entries / get_entry_dict() returned.

Adds a short v1/v2 snippet to "Accessing the library", plus a note that values may not be strings after middleware (e.g. SeparateCoAuthors):

entries = [dict(entry) for entry in library.entries]
entries_dict = {entry["ID"]: entry for entry in entries}

dict(entry) relies on #631. Snippet verified; sphinx-build -W and pre-commit pass.

Refs #629