docs: expand README with comprehensive project overview

[Copilot]

The README provided minimal description of the project, making it hard for newcomers to quickly understand what the Control Plane is, what it does, and how it's structured.

Changes

• About — Adds a second paragraph clarifying the Kubernetes Operator pattern foundation and key integrations (Kong Gateway, Keycloak)
• Key Features — New section listing 8 capabilities: API lifecycle, approval workflows, gateway/identity integration, org management, secret/file management, notifications, and declarative config
• Architecture — New section with component tables organized into four categories: Operators (9), API Servers (4), Shared Libraries (2), CLI Tools (1) — each linking to its subdirectory
• Technology Stack — New table summarizing technologies across language, framework, HTTP, gateway, identity, storage, testing, and deployment

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.