Update documentation for ArFoundation and Vuforia.

[timbotimbo]

Description

Updates the readme for both ARFoundation and Vuforia.

• I removed the Android instructions to manually include all libraries.
• Instruction for unityLibrary:xrmanifest.androidlib error.
• Added iOS section for Vuforia.
• Added a warning about the Flutter 3.22 crash on Android. [Flutter 3.22 & newer] AR crashes on Android < 13 - NoClassDefFoundError: Landroid/window/OnBackInvokedCallback #957

This replaces PR #906 and #911.

This should help with issues like

#844, #689, #904 and multiple other issues with the Vuforia label

@Ahmadre Since you recently had issues with Vuforia, maybe you can review this.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[timbotimbo]

Closed because of a lack of maintainers/contributers.