docs: document DateFormat source #1463

[rednikisfun]

Summary

• Documents that FormBuilderDateTimePicker.format uses DateFormat from the intl package.
• Adds a README example showing the required package:intl/intl.dart import and a custom date format.
• Adds a direct API-doc link to the DateFormat class so users can find the type definition from generated docs.

Closes #1463

Verification

• dart format .
• flutter analyze
• flutter test

Notes

• This keeps the change documentation-only rather than re-exporting DateFormat, which avoids changing the public API surface.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (571c43c) to head (fbd6c4b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1509   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          21       21           
  Lines         853      853           
=======================================
  Hits          775      775           
  Misses         78       78           

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[deandreamatias]

Thanks!