docs: Enhance Apple Sign-In setup and configuration documentation

[Zfinix]

Problem

The Sign in with Apple docs lacked screenshots for the Apple Developer Portal setup, had no clear separation between what iOS/macOS, Android, and Web each require, and didn't explain key gotchas (raw .p8 key vs pre-generated JWT, HTTPS-only redirect URIs, YAML indentation). There was also no troubleshooting page.

Solution

• Setup page — Added 8 screenshots for the App ID, Service ID, and key creation steps. Added a platform requirements table, warnings for common mistakes, and the Android intent filter snippet.
• Configuration page — Converted parameter tables to bullet lists to match other provider pages. Added an AppleIdpConfig parameter reference.
• Troubleshooting page (new) — Setup checklist and 10 documented failure scenarios with cause and fix for each.