docs: add submit button state example #1490

[rednikisfun]

Summary

• Adds a new SubmitButtonForm example that enables/disables the submit button from FormBuilder state.
• Demonstrates conditional validation by requiring email only when the updates switch is enabled.
• Wires the example into the example app navigation and README docs.

Closes #1490

Verification

• dart format .
• flutter analyze
• flutter analyze example
• flutter test

Notes

• Left pubspec.lock and example/pubspec.lock unchanged. On my local Flutter SDK, flutter pub get --enforce-lockfile wants to update unrelated transitive meta and test_api versions, so I kept this PR scoped to docs/example changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (38a24f6) to head (f080271).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1508   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          21       21           
  Lines         853      853           
=======================================
  Hits          775      775           
  Misses         78       78           

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[deandreamatias]

Thanks a lot for this contribution! I will merge