docs: add usage example for retrieval_core #20

docs/usage/retrieval_core.md

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+# Usage Example: retrieval_core
+
+`retrieval_core` is a probabilistic task used for document indexing and query estimation.
+
+## Example Script
+
+```python
+from chuck.tasks.retrieval_core.task import solve, generate
+
+# 1. Generate sample data (documents and queries)
+payload = generate(count=100, seed=42)
+
+# 2. Execute the retrieval solver
+# The payload contains 'docs' and 'queries'
+results = solve(payload)
+
+# 3. View Probabilistic Results
+print(f"Confidence: {results['confidence']}")
+print(f"Query Hits: {results['query_hits']}")
+print(f"Top Term: {results['top_term']}")
+```
+
+# Outputs
+```text
+Confidence: 0.875
+Query Hits: {'eta': 68, 'kappa': 76, 'alpha': 72, 'delta': 70}
+Top Term: epsilon
+```