docs(releases): rewrite upgrade guide with AI-friendly format

- Add AI coding assistant prompt suggestion for automated migration
- Reorder sections to match the quick checklist sequence
- Add cross-reference links to docs pages in every section
- Expand checklist with missing items (is_terminated, self-transitions,
  __repr__, new exports)
- Add Before/After examples to all sections (StateChart vs StateMachine,
  New public exports)
- Clarify StateChart recommendation: about SCXML defaults, not features
- Note current_state.final still works for flat machines
- Recommend class-level listener declarations for new code
- Explain SCXML rationale for discarding unmatched events
- Add tip about error-as-events migration path
- Show concrete trap state example in strict_states Before block
- Condense "New features overview" to single paragraph with link

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: fgmacedo