docs(transitions): rewrite with coherent examples and narrative flow

Reorganize transitions.md with progressive narrative and consistent
order-processing domain across all examples.

Narrative order: declaring → parameters table → combining with | →
from_()/from_.any() → self/internal transitions → eventless →
cross-boundary → priority.

Key improvements:
- All examples use order/e-commerce domain (replacing disconnected
  LotR and generic examples)
- Self and internal transitions unified in one section with practical
  contrast (RetryOrder vs OrderCart)
- from_.any() with equivalent compound state alternative, side-by-side
  diagrams showing both approaches
- Equivalent state topology between flat and compound examples

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

Author: fgmacedo