Document the models package, drop duplicated JSON blobs from docstrings

[sigma67]

Part of #307. Base branch: ytmusicapi-2. Blocked until the Phase 2 issues have landed.

Scope

• Autodoc the ytmusicapi/models package so every response model appears in the API reference with its fields and types.
• Replace the inline JSON example blocks in mixin docstrings with :class: references to the models. Those blobs are currently the only specification of the return shapes and they drift; once the models are authoritative, the duplication is a liability.
• Keep a short example where it genuinely aids comprehension — the goal is removing the duplicated field lists, not removing all examples.

sphinx-autodoc-typehints is already a dev dependency, so the machinery is in place.

Definition of done

• Every public method's return type resolves to a documented model in the rendered docs.
• No mixin docstring still lists fields that are also declared on a model.