Docs: multi-step wizard recipe + example for the Flask adapter

[fsecada01]

Summary

Port the multi-step wizard recipe (#29, #30, #31) to the Flask adapter:
a wizard example under examples/ (or examples/flask_example.py-adjacent)

• a Flask subsection in docs/examples/wizard.md.

Why this waits until last

The Flask adapter (#5, landed in PR #19) is the newest and least
battle-tested of the four — it doesn't yet have the same volume of tests/
real usage as FastAPI, Litestar, or Django. Writing a multi-step wizard
recipe against it is a reasonable way to stress-test the adapter's
event/state handling, but it should happen after the pattern is already
proven twice over (FastAPI #29, Litestar #30) so any adapter-specific bugs
surfaced by the wizard are easier to isolate.

Deliverable

• Flask wizard example (3-step scenario, consistent with Docs: multi-step wizard recipe + example for the FastAPI adapter #29/Docs: multi-step wizard recipe + example for the Litestar adapter #30).
• Flask subsection in docs/examples/wizard.md covering anything
  Flask-specific (routing, CSRF — Flask has no built-in CSRF handling in
  the adapter today either, same caveat as FastAPI/Docs: multi-step wizard recipe + example for the FastAPI adapter #29's A4 note).

Sequencing

Last in the wizard-recipe series: after #30 (Litestar) and #31 (Django).