docs: enhance README with comprehensive documentation

[zchalmers]

This PR significantly enhances the README.md with:

Improvements:

• Fixed incorrect build tool reference (was Maven, now correctly shows Gradle)
• Added comprehensive feature list with emojis for better readability
• Detailed architecture section covering backend, frontend, and external services
• Complete installation and setup instructions with code examples
• API endpoint documentation with request/response examples
• Project structure overview
• Expanded technologies used section
• Configuration guide including carrier email gateways
• Troubleshooting section
• Security best practices
• Development workflow instructions

What's New:

• Better organized sections with clear hierarchy
• Visual formatting improvements
• Usage examples and screenshots description
• Testing and development instructions
• API documentation reference

The enhanced README provides developers and users with all necessary information to understand, set up, and use the Weather Notification Service.