Update internationalization documents and gen_l10n example.

[amir14a]

Update internationalization documents and gen_l10n example to align with deprecation of synthetic packages. From flutter 3.32.0 breaking changes, localized messages are generated into source, not a synthetic package. This PR should be merged in the future stable release. Issues fixed by this PR (if any):

• https://github.com/flutter/website/issues/11765

Relative links:

• https://groups.google.com/g/flutter-announce/c/ZWtR2h6QAZk
• https://flutter.dev/to/flutter-gen-deprecation
• https://github.com/flutter/flutter/issues/102983
• https://github.com/flutter/flutter/pull/157934

Presubmit checklist

• [ ] This PR is marked as draft with an explanation if not meant to land until a future stable release.
• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.