Complete User Guide documentation and expand IFrame API guides

[4rayaditya]

Comprehensive documentation improvements including:

User Guide Enhancements:

• Expanded Google Calendar integration guide (10→219 lines)
• Complete keyboard shortcuts reference with platform-specific notes (23→212 lines)
• Optimized Basic and Advanced user guides for clarity

Developer Guide Additions:

• Mobile Feature Flags: Complete guide with 20+ flags and examples (3→378 lines)
• Mobile Google Auth: Firebase setup and troubleshooting (25→119 lines)
• Mobile Dropbox Integration: Full setup guide (30→160 lines)
• IFrame API: 4 comprehensive guides (Quick Start, Workflows, Migration, Best Practices)
• 80+ IFrame API improvements across Commands, Events, and Functions