docs: create comprehensive unified architecture document

[NkwaTambe]

Descritpion

This commit introduces a complete and detailed unified architecture document that serves as a single source of truth for the entire platform.

The document was created by expanding on an initial draft and filling in numerous missing sections based on an analysis of the codebase and deployment configurations.

Key additions include:

• Detailed explanations for all custom components, including the Reporting Plugin, Fineract Config CLI, Keycloak Theme, and custom adorsys extension modules.
• A complete "Deployment Strategy" section explaining the GitOps workflow with ArgoCD, Kubernetes, and Terraform.
• A "Repositories Overview" section listing all relevant internal and upstream repositories.
• A full set of architecture diagrams created using Mermaid syntax, including:
  • High-Level System Architecture
  • Component Interaction Diagram
  • Authentication & Authorization Flow
  • Deployment & Network Topology
  • Data Flow for Key Business Operations