Add NetworkX graph visualization example to README

[chigwell]

User Story
As a developer using the eknowledge package,
I want a visual example of graph generation in the README
so that I can quickly understand how to visualize extracted relationships.

Background
The current README's code example only shows text-based graph output ([{'from': 'quick brown fox', 'relationship': 'interacts_with', 'to': 'lazy dog'}]), which fails to demonstrate practical visualization. This creates friction for users who need to validate graph structures visually. The execute_graph_generation function in main.py already returns graph data - we should extend the example to show NetworkX-based visualization.

Acceptance Criteria

• Modify README.md to include a graph visualization example after the existing text output
• Add NetworkX/Matplotlib code to:
  1. Convert the extracted relationships into a NetworkX DiGraph
  2. Generate a matplotlib figure with node labels and layout
  3. Save visualization as example_graph.png
• Ensure the example:
  • Includes import networkx as nx and import matplotlib.pyplot as plt
  • Uses distinct colors for nodes/edges
  • Shows both text output AND displayed/saved visualization
• Verify the visualization renders correctly when users copy-paste the example
• Update installation instructions to include pip install networkx matplotlib