New docs page describing the changes needed to upgrade between
major versions, with a detailed 3.x to 4.0 section covering CLI,
Python API and packaging changes, plus brief sections for the
2.x to 3.0 and 1.x to 2.0 upgrades.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd
