Translate incompatible PHP features (INCOMPATIBLE_PHP_FEATURES.md) file to English

[FunkyOz]

Summary

Adds an English translation of the AOT/PHP incompatibility documentation while preserving the existing Chinese version.

Changes

• Add docs/en/INCOMPATIBLE_PHP_FEATURES.md.
• Translate all compatibility sections and technical details into English.
• Update README references to point to the English document.
• Link the PHP incompatibility classification document to the English checklist.
• Preserve TypePHP-specific terminology and documented compatibility boundaries.

Testing

Documentation-only change.

• Verified that all sections and list items from the source document are present.
• Verified that the updated relative links resolve correctly.
• Confirmed that git diff --check passes.