docs: add semantic cache doc to explain how to use in-memory and milvus in the config

Signed-off-by: Huamin Chen <hchen@redhat.com>

Author: rootfs

website/docs/getting-started/semantic-cache.md

@@ -0,0 +1,191 @@
+# Semantic Cache
+
+Semantic Router provides intelligent caching that understands request similarity using semantic embeddings. Instead of exact string matching, it identifies semantically similar queries to serve cached responses, reducing latency and LLM inference costs.
+
+## Architecture
+
+```mermaid
+graph TB
+    A[Client Request] --> B[Semantic Router]
+    B --> C{Cache Enabled?}
+    C -->|No| G[Route to LLM]
+    C -->|Yes| D[Generate Embedding]
+    D --> E{Similar Query in Cache?}
+    E -->|Hit| F[Return Cached Response]
+    E -->|Miss| G[Route to LLM]
+    G --> H[LLM Response]
+    H --> I[Store in Cache]
+    H --> J[Return Response]
+    I --> K[Update Metrics]
+    F --> K
+    
+    style F fill:#90EE90
+    style I fill:#FFB6C1
+```
+
+## Backend Options
+
+### Memory Backend (Development)
+- **Use case**: Development, testing, single-instance deployments
+- **Pros**: Fast startup, no external dependencies
+- **Cons**: Data lost on restart, limited to single instance
+
+### Milvus Backend (Production/Persistent)
+- **Use case**: Production, distributed deployments
+- **Pros**: Persistent storage, horizontally scalable, high availability
+- **Cons**: Requires Milvus cluster setup
+
+## Configuration
+
+### Memory Backend
+```yaml
+semantic_cache:
+  enabled: true
+  backend_type: "memory"
+  similarity_threshold: 0.8
+  max_entries: 1000
+  ttl_seconds: 3600
+```
+
+### Milvus Backend
+```yaml
+semantic_cache:
+  enabled: true
+  backend_type: "milvus"
+  backend_config_path: "config/cache/milvus.yaml"
+  similarity_threshold: 0.8
+  ttl_seconds: 3600
+```
+
+## Testing Cache Functionality
+
+### Test Memory Backend
+
+Start the router with memory cache:
+```bash
+# Run the router
+make run-router
+```
+
+Test cache behavior:
+```bash
+# Send identical requests to see cache hits
+curl -X POST http://localhost:8080/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "auto",
+    "messages": [{"role": "user", "content": "What is machine learning?"}]
+  }'
+
+# Send similar request (should hit cache due to semantic similarity)
+curl -X POST http://localhost:8080/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "auto", 
+    "messages": [{"role": "user", "content": "Explain machine learning"}]
+  }'
+```
+
+### Test Milvus Backend
+
+Start Milvus container:
+```bash
+make start-milvus
+```
+
+Update configuration to use Milvus:
+```bash
+# Edit config/config.yaml
+sed -i 's/backend_type: "memory"/backend_type: "milvus"/' config/config.yaml
+sed -i 's/# backend_config_path:/backend_config_path:/' config/config.yaml
+```
+
+Run with Milvus support:
+```bash
+# Run the router
+make run-router
+```
+
+# Stop Milvus when done
+```bash
+make stop-milvus
+```
+
+## Monitoring Cache Performance
+
+### Available Metrics
+
+The router exposes Prometheus metrics for cache monitoring:
+
+| Metric | Type | Description |
+|--------|------|-------------|
+| `llm_cache_hits_total` | Counter | Total cache hits |
+| `llm_cache_misses_total` | Counter | Total cache misses |
+| `llm_cache_operations_total` | Counter | Cache operations by backend, operation, and status |
+| `llm_cache_operation_duration_seconds` | Histogram | Duration of cache operations |
+| `llm_cache_entries_total` | Gauge | Current number of cache entries |
+
+### Cache Metrics Dashboard
+
+Access metrics via:
+- **Metrics endpoint**: `http://localhost:9190/metrics`
+- **Built-in stats**: Available via cache backend `GetStats()` method
+
+Example Prometheus queries:
+```promql
+# Cache hit rate
+rate(llm_cache_hits_total[5m]) / (rate(llm_cache_hits_total[5m]) + rate(llm_cache_misses_total[5m]))
+
+# Average cache operation duration
+rate(llm_cache_operation_duration_seconds_sum[5m]) / rate(llm_cache_operation_duration_seconds_count[5m])
+
+# Cache operations by backend
+sum by (backend) (rate(llm_cache_operations_total[5m]))
+```
+
+### Cache Performance Analysis
+
+Monitor these key indicators:
+
+1. **Hit Ratio**: Higher ratios indicate better cache effectiveness
+2. **Operation Latency**: Cache lookups should be significantly faster than LLM calls  
+3. **Entry Count**: Monitor cache size for memory management
+4. **Backend Performance**: Compare memory vs Milvus operation times
+
+## Configuration Best Practices
+
+### Development Environment
+```yaml
+semantic_cache:
+  enabled: true
+  backend_type: "memory"
+  similarity_threshold: 0.85  # Higher threshold for more precise matching
+  max_entries: 500           # Smaller cache for testing
+```
+
+### Production Environment
+```yaml
+semantic_cache:
+  enabled: true
+  backend_type: "milvus"
+  backend_config_path: "config/cache/milvus.yaml"
+  similarity_threshold: 0.8   # Balanced threshold
+```
+
+### Milvus Production Configuration
+```yaml
+# config/cache/milvus.yaml
+connection:
+  host: "milvus-cluster.prod.example.com" # Replace with your Milvus cluster endpoint
+  port: 443
+  auth:
+    enabled: true
+    username: "semantic-router" # Replace with your Milvus username
+    password: "${MILVUS_PASSWORD}" # Replace with your Milvus password
+  tls:
+    enabled: true
+
+development:
+  drop_collection_on_startup: false  # Preserve data
+  auto_create_collection: false      # Pre-create collections
+```