docs: update Architecture section with Mermaid diagram

[shlok926]

Description

This PR updates the README.md to enhance the visualization of the project's architecture.

Changes

• Replaced the basic ASCII architecture box with a proper, end-to-end Mermaid diagram.
• The new diagram clearly illustrates the data flow through the four main components:
  1. Presentation Layer (CLI)
  2. Business Logic Layer (Orchestrator & Scanners)
  3. Data Access Layer (Models & Output Manager)
  4. Infrastructure Layer (Cloud SDKs)
• Visually maps out the connections to external Cloud APIs (AWS, Azure, GCP).