Add custom vector store implementation notebook

Create comprehensive notebook demonstrating how to implement and register custom vector stores with GraphRAG as a plug-and-play framework. Includes:

- Complete implementation of SimpleInMemoryVectorStore
- Registration with VectorStoreFactory
- Testing and validation examples
- Configuration examples for GraphRAG settings
- Advanced features and best practices
- Production considerations checklist

The notebook provides a complete walkthrough for developers to understand and implement their own vector store backends.

Co-authored-by: jgbradley1 <654554+jgbradley1@users.noreply.github.com>

Author: Copilot