Consolidate folder structure and conventions documentation

[josbeir]

Merged cakephp-folder-structure.md and conventions.md into a single comprehensive Structure & Conventions guide.

Changes:

• Combined two separate pages (798 lines → 369 optimized lines) into /intro/conventions
• Added VitePress enhancements for better readability:
  • 6 collapsible sections to reduce initial page length (~40%)
  • 3 tabbed code groups showing correct vs incorrect examples
  • 6 alert containers (TIP/WARNING/NOTE) for critical information
• Added documentation links to all folder descriptions in src/ directory table
• Updated TOC to reflect single consolidated entry
• Removed duplicate content and legacy HTML tables

Benefits:

• Single source of truth for CakePHP conventions
• Improved navigation with consistent cross-references
• Progressive disclosure reduces cognitive load for new developers
• Better visual learning through side-by-side code comparisons

All markdown validated with zero errors.