Improve the Apple sign-in documentation

[Swiftaxe]

Goal

Improve the Apple sign-in documentation to make it easier to follow.
Part of #503.

Tasks


• Fact-check against the latest Serverpod version.
• Rewrite the setup guide to be explicit, unambiguous, and ensure all steps are documented.
• Add clear and complete examples for IDP portal, client-side, and server-side implementations.
• Rename the existing "Configurations" page to "Customizations" for clarity.
• Move advanced use cases to the "Customizations" page to keep the primary guide focused.
• Add a "Publishing to production" section that details specific steps for production.
• Create a "Troubleshooting" page that lists common errors, their causes, and solutions.


Definition of done

• A developer can follow the guide from start to finish and successfully get it working locally and on Serverpod Cloud.
• The process is easy to complete without requiring extra research.
• Every step is clearly documented.
• All required interactions with an Identity provider's portal are documented with clear and up-to-date screenshots.