Update README with clearer project explanation and quick start guide

[adilhafeez]

Summary

• Added a new "What is Plano?" section with a clear project description and key capabilities table
• Added a "Quick Start" section with step-by-step installation and usage instructions
• Reorganized the README to help new users understand what Plano is and how to get started in under 5 minutes

Changes

New Sections Added:

1. What is Plano? - Explains Plano's purpose as an AI-native proxy and data plane for agentic apps
2. Key Capabilities - Table showcasing orchestration, LLM routing, signals, guardrails, memory, and observability
3. Why Plano? - Explains the benefits of decoupling concerns and being language/framework/model agnostic
4. Quick Start - Step-by-step guide with installation commands for macOS, Linux, and Docker

Improvements:

• Clearer value proposition for new users
• Concrete installation and usage examples
• Better organization with horizontal rules separating major sections
• Links to documentation for next steps

Impact

This update makes the README more accessible to first-time visitors by:

• Explaining what Plano is before diving into how to use it
• Providing a working "hello world" example in the Quick Start
• Highlighting the key differentiators in an easy-to-scan format