docs: Complete README rework for MCP Nexus platform

- Repositioned project as comprehensive MCP platform (not just debugging)
- Added clear platform-first branding and future-oriented messaging
- Reorganized structure: Quick Start  Transport  Tools  Integration  Development
- Enhanced tool categorization with planned expansions (System Admin, Development, etc.)
- Added professional architecture documentation and contribution guidelines
- Fixed missing 'type: stdio' in MCP configuration examples for clarity
- Improved troubleshooting and configuration sections
- Added comprehensive integration examples for Cursor and other MCP clients

The README now positions MCP Nexus as an extensible platform for diverse AI tool integration.

Author: CapulusCodeNinja