Add architecture documentation with diagrams

[ahmed-sekka]

Description

Create comprehensive architecture documentation with visual diagrams to help new contributors understand the codebase.

Proposed content

1. High-level architecture diagram

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   CLI       │────▶│   Core      │────▶│   Output    │
│  (Typer)    │     │  Pipeline   │     │  (Storage)  │
└─────────────┘     └─────────────┘     └─────────────┘
                           │
              ┌────────────┼────────────┐
              ▼            ▼            ▼
        ┌─────────┐  ┌─────────┐  ┌─────────┐
        │  OCR    │  │ Chunker │  │ Vector  │
        │ Engine  │  │         │  │  Store  │
        └─────────┘  └─────────┘  └─────────┘

2. Data flow diagram

• Document input → Loading → OCR (optional) → Chunking → Output

3. Module dependency graph

• Show relationships between src/core/ modules

4. Configuration hierarchy

• CLI > Environment > YAML > Defaults

Deliverables

• Update docs/ARCHITECTURE.md
• Add Mermaid diagrams (GitHub-rendered)
• Document key design decisions
• Add "Contributing to X module" sections