Add Mermaid architecture diagrams to README

[ericsciple]

👉 Rendered 👈

[Copilot]

Pull request overview

This PR adds Mermaid-based architecture diagrams to the README to provide a visual overview of the extension's architecture. The diagrams complement the existing detailed architecture documentation in docs/project-architecture.md by offering an at-a-glance understanding of the system's components and relationships.

• Adds two Mermaid diagrams: a high-level architecture overview and a key components diagram
• Includes a helpful comment directing users to install the "Markdown Preview Mermaid Support" extension for VS Code
• Uses color coding to highlight different component types (extension, language server, GitHub API, authentication, management, validation)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.