Add detailed documentation explaining Brainy's 4-index architecture:
- MetadataIndex: inverted indexing with temporal bucketing (v3.41.0)
- HNSWIndex: hierarchical vector similarity search
- GraphAdjacencyIndex: O(1) bidirectional relationship traversal
- DeletedItemsIndex: soft-delete tracking
Includes explanation of UnifiedCache for coordinated memory management
across all indexes, integration patterns, performance characteristics,
and best practices for developers.
Updated overview.md to reference the new index architecture documentation.
