Add MongoDB aggregation pipeline optimization blog post

Comprehensive tutorial covering:
- Performance optimization strategies for complex analytics
- Pipeline stage ordering and memory management
- Advanced aggregation patterns with window functions
- Index optimization and bottleneck identification
- Time-series analysis and cohort analytics
- RFM analysis for customer segmentation
- SQL-familiar syntax through QueryLeaf integration

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rathboma