docs: provide interactive demonstration on README

[alestiago]

Is your feature request related to a problem? Please describe.
Inspired by #95, users should be able to run the example to quickly evaluate the package.

Describe the solution you'd like
I would like users to be able to try out an application using formz with a single click. Ideally, without cloning and having to run the example themselves.

The solution I'l like is to have an image within the README that when clicked opens a hosted page with the running example.

Describe alternatives you've considered
Another option is to wait until pub.dev support hosting the examples for users to interact with.

Additional context

• See Modal bottom sheet, that had an interactive demonstration.