Add modern quickstart showcasing type-hint based validation

[amol-]

A modern default quickstart should show the current recommended TurboGears path clearly:

• Kajiki templates
• server-rendered pages
• standard @validate() validation with type hints
• a small, polished first-run demo that is easy to delete

The generated app should be Kajiki-only and should no longer expose Mako, Jinja, or
Genshi quickstart variants. The goal is to make the default output smaller, clearer, and
easier for new users to understand.

• Use Bootstrap 5 from a CDN, with no frontend build step.
• Include a small SQLAlchemy-backed Todo demo when SQLAlchemy is enabled
• Use htmx to showcase realtime updates of the Todo list