Add graph visualization feature

[matheu-s]

Changes introduced:

Adds visualize_graph.py, that generates an HTML visualization of the knowledge graph (graph_chunk_entity_relation.graphml):

These changes are important, as they help the user to identify problems with unconnected nodes and easily debug the processed knowledge graph.

• It adds the weights on the edge label of connected nodes.
• All nodes have a tooltip with category, description and chunk source.

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How to test:

1. Process a sample.txt or add a ready graph_chunk_entity_relation.graphml file.
2. Run visualize_graph.py.
3. Open knowledge_graph.html.

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Obs: If preferred, the functionality could be implemented to automatically generate the visualization HTML, it if executes after graph creation: https://github.com/BUPT-GAMMA/PathRAG/blob/2785a12de7519b1869483b31ed8716179e7bc03e/PathRAG/storage.py#L259-L260