docs: Document sign-in widget localization

[marcelomendoncasoares]

Summary

SignInLocalizationProvider lets an app replace the English text of the authentication widgets, but nothing in the docs mentioned it.

Changes

• Add a "Localization" section to the authentication UI components page. It is a top-level section because the provider applies to every authentication widget, not only SignInWidget.
• List the text objects and what each one covers before the example.
• Add one complete example translating the shared texts, the email flow, the password requirements, and every provider button with explicit English texts.
• Explain the fallback: anything left out keeps the built-in English text, and defaults.copyWith covers partial translations.
• State that the provider does not resolve Flutter's Locale, and that the app has to rebuild it when the locale changes.
• Note that the Firebase provider ships a controller rather than a finished UI, so it is not covered by these text objects.
• Fix the sentence case of the existing "Disabling providers" heading. The slug is unchanged.

Reference

• feat(auth): Implement localization for sign in widget serverpod#4711
• SignInWidget internationalization serverpod#4496

[developerjamiu]

The PR description says the example translates into Brazilian Portuguese, but all the strings in the diff are English. The English version reads well as an API reference, so maybe just update the description, or give the copyWith example one non-English string (e.g. signIn: 'Entrar') so the translation point shows.

[developerjamiu]

Changes looks good! Please check the added (minor) comment before merge