2.0 migration guide

[sigma67]

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

Scope

Write the 2.0 migration guide covering everything in #307 that a consumer needs to act on:

• Key presence changed. Every declared field is now always a key; a field the API did not return is present with value None rather than absent. Code branching on if "count" in playlist: or on .get("count") truthiness must compare against None instead. This is the one deliberate break in the effort and needs the most prominent treatment, with before/after examples.
• pydantic is now a runtime dependency alongside requests.
• Attribute access is available. song.title works; song["title"] continues to work.
• Anything the Phase 2 issues turned up that changed shape in passing.

Definition of done

• A user upgrading from 1.x can find every required change in one place.
• Linked from the README and the changelog for the 2.0 release.