docs: Rename Facebook configuration page to customizations and fix UI API

[Zfinix]

Summary

Aligns the Facebook sign-in docs with the other migrated providers (Google, Apple, GitHub) and corrects the UI customization API.

• Renames 02-configuration.md to 02-customizations.md and updates its title, sidebar_label, and description to match the Google/GitHub customizations pages. Updates the inbound link from the setup page.
• Fixes the FacebookSignInWidget example on the UI customization page, which referenced an API that does not exist in serverpod_auth_idp_flutter_facebook:
  • The button label prop is type (a FacebookButtonText), not text.
  • Removes the non-existent FacebookButtonType.standard / .icon (there is no FacebookButtonType enum).
  • Corrects enum values to signinWith and signupWith.
  • FacebookButtonSize also has small; FacebookButtonShape only has rectangular and pill.
  • Documents the minimumWidth prop.

Verified against serverpod_auth_idp_flutter_facebook 3.5.0-beta.10 source.

Part of #503, and a step toward #507.