docs: add comprehensive project documentation

Why:
- Need documentation for team onboarding and reference
- Need technical details for strategy implementations
- Need API reference for developers

Changes:
- README.md: Documentation index and quick start guide
- api-reference.md: All 10 endpoints with examples
- architecture.md: System structure and design decisions
- configuration.md: All environment variables and parameters
- data-flow.md: How data moves through the system
- evaluation-metrics.md: How scoring works with weights
- strategy-technical-details.md: Deep dive into each strategy's implementation

docs/README.md

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+# Documentation Index
+
+Complete documentation for the RAG Chunking Benchmarker.
+
+---
+
+## Quick Start
+
+1. **New to the project?** Start with [Architecture Overview](architecture.md)
+2. **Want to understand strategies?** Read [Strategy Technical Details](strategy-technical-details.md)
+3. **Need to use the API?** Check [API Reference](api-reference.md)
+4. **Configuring the system?** See [Configuration Guide](configuration.md)
+5. **Understanding results?** Read [Evaluation Metrics](evaluation-metrics.md)
+6. **Curious about data flow?** See [Data Flow](data-flow.md)
+
+---
+
+## Documentation Files
+
+| File | Purpose | Audience |
+|------|---------|----------|
+| [architecture.md](architecture.md) | System structure and design | New team members |
+| [strategy-technical-details.md](strategy-technical-details.md) | Deep dive into each strategy | Engineers |
+| [api-reference.md](api-reference.md) | All endpoints documented | Developers |
+| [configuration.md](configuration.md) | Settings and environment variables | DevOps |
+| [evaluation-metrics.md](evaluation-metrics.md) | How scoring works | Data scientists |
+| [data-flow.md](data-flow.md) | How data moves through the system | Engineers |
+| [chunking_strategies.md](chunking_strategies.md) | High-level strategy overview | Everyone |
+| [phases.md](phases.md) | Implementation phases | Project managers |
+| [tasks.md](tasks.md) | Task tracking | Developers |
+
+---
+
+## Architecture at a Glance
+
+```
+┌─────────────────────────────────────────────────────────────────┐
+│                    RAG Chunking Benchmarker                     │
+├─────────────────────────────────────────────────────────────────┤
+│                                                                 │
+│  Documents → Parser → Chunking → Embedding → Qdrant           │
+│                                                                 │
+│  Questions → Query Pipeline → LLM → Answers                    │
+│                                                                 │
+│  Answers → Evaluation → Metrics → Reports                      │
+│                                                                 │
+└─────────────────────────────────────────────────────────────────┘
+```
+
+---
+
+## Key Concepts
+
+### Chunking Strategies
+
+| Strategy | Splitting | Best For |
+|----------|-----------|----------|
+| fixed_size | Token count | Baseline comparison |
+| recursive | Structural boundaries | Structured documents |
+| semantic | Similarity thresholds | Topic shifts |
+| contextual_retrieval | LLM-enriched tokens | Retrieval quality |
+| semantic_parent_child | Paragraph clusters | Context needed |
+
+### Evaluation Metrics
+
+| Metric | What It Measures |
+|--------|------------------|
+| Context Relevance | Did we find the right information? |
+| Answer Similarity | Did we produce the right answer? |
+| Faithfulness | Is the answer trustworthy? |
+| Hallucination | Did we invent information? |
+
+---
+
+## API Endpoints
+
+| Endpoint | Method | Purpose |
+|----------|--------|---------|
+| `/documents` | GET/POST | Manage documents |
+| `/documents/{id}/process` | POST | Run chunking strategies |
+| `/queries` | POST | Ask questions |
+| `/benchmarks` | POST | Run comparisons |
+| `/benchmarks/{id}/report` | GET | View results |
+| `/experiments` | GET | List experiments |
+
+---
+
+## Configuration
+
+All settings via `.env` file. See [Configuration Guide](configuration.md) for details.
+
+Key settings:
+- `OPENAI_API_KEY` - Required for embeddings and LLM
+- `CHUNK_SIZE` - Target tokens per chunk (default: 512)
+- `SEMANTIC_THRESHOLD` - Similarity threshold (default: 0.5)
+
+---
+
+## Further Reading
+
+- [Phases](phases.md) - Implementation roadmap
+- [Tasks](tasks.md) - Task tracking
+- [ADRs](adr/) - Architectural Decision Records