docs: add MongoDB index management section to README

Added comprehensive documentation on how arrest manages MongoDB collection indexes:
- Automatic index verification on collection first access
- Optional auto-creation with createIndexes flag
- How to define custom indexes via getIndexes() method
- Support for unique, sparse, TTL, text, and compound indexes
- Default behavior for custom ID fields
- Index verification process and logging

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

Author: GianfrancoFrau