docs: add migration guide from pytransitions (#584)

New "Coming from pytransitions" page comparing the two libraries with
side-by-side code examples covering: state/transition definitions (flat
and compound), events, callbacks with dependency injection, conditions,
completion events (done.state), invoke, async, diagrams, unified API vs
class composition, model integration, listeners, error handling, and
validations. Includes a comprehensive feature matrix with cross-references.

Adds a "How to" toctree section and a mention in the v3.0.0 release notes.

Author: fgmacedo