docs: add comprehensive development setup guide
Updates README.md with complete setup instructions:
- Architecture overview
- Prerequisites list
- Step-by-step dev environment setup
- Environment variables reference (required vs optional)
- Telegram bot setup via BotFather
- Webhook vs polling mode explanation
- Docker services management commands
- Testing instructions
- Project structure overview
- Milestone progress tracker
- Troubleshooting section
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>