Add optional graph visualization cells to Graph-RAG quickstart notebook

[Subhi-Sadiyev]

📌 Description

This PR adds optional graph knowledge visualization cells to the Graph-RAG quickstart notebook (graph_rag_with_milvus.ipynb).

The visualizations help users understand how entities and relations connect in the Graph-RAG workflow, using nano_dataset as example.

These additions are purely educational and no changes to retrieval logic, data flow or Milvus behavior is made.

Closes #1566

✅ Changes

• Feature added:
  Added two optional visualization cells:
  1. Full knowledge graph visualization (after entityid_2_relationids / relationid_2_passageids)
  2. Final expanded subgraph visualization (after relation_candidate_ids)
• Bug fixed: n/a
• Refactor: n/a
• Other (explain): n/a

🔬 How to Test

1. Open tutorials/quickstart/graph_rag_with_milvus.ipynb
2. Run all cells in order — both new visualizations render inline (matplotlib + networkx)

📝 Checklist

• Code compiles and runs
• Linting passes (Check jupyter notebooks: pip install "black[jupyter]" > black {file_or_directory})
• PR includes relevant docs or comments
• Rebased on the latest main (no merge commits or conflicts)

🧼 Commit Hygiene

✔️ Follow best practices:

• Small, focused commits with clear messages
• Group related changes together
• Avoid large monolithic commits
• Use meaningful commit messages (e.g., fix:, feat:, refactor:)

👀 Reviewer Notes

• Please verify cell placement (immediately after the noted data structures are built)
• Confirm that visualization runs inline only (no file writes)

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📎 Related Issues, PRs, or Docs

• Related PR: #
• Issue: #

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[sre-ci-robot]

Welcome @Subhi-Sadiyev! It looks like this is your first PR to milvus-io/bootcamp 🎉