INTPYTHON-501 INTPYTHON-784 Create Graph Visualizations for GraphRAG (#235)

[INTPYTHON-501](https://jira.mongodb.org/browse/{INTPYTHON-501})
[INTPYTHON-784](https://jira.mongodb.org/browse/{INTPYTHON-784})

## Summary

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the JIRA ticket, still place it in the Pull Request as you should not
make the reviewer do digging for a basic summary. -->

This PR adds graphical visualization of our MongoDBGraphStore, used for
Knowledge Graphs and Graph RAG. It also adds an example notebook, and
improvements to the prompting.

## Changes in this PR

<!-- What changes did you make to the code? What new APIs (public or
private) were added, removed, or edited to generate the desired outcome
explained in the above summary? -->

* New methods `view` and `to_networkx` methods to MongoDBGraphStore.
* Our first example notebook in
langchain-mongodb/libs/langchain-mongodb/examples/GraphRAG.ipynb.
* Improvements to the prompt examples.
* A new optional dependency group "viz".

## Test Plan

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that. If you didn’t introduce unit tests, explain why. All code should
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Unit tests were added for the new methods. Typing tests were run. The
notebooks was demonstrated numerous times to the maintainers and
stakeholders.

## Checklist

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### Checklist for Author

- [x] Did you update the changelog (if necessary)?
- [x] Is the intention of the code captured in relevant tests?
- [ ] If there are new TODOs, has a related JIRA ticket been created?
- [x] Has a MongoDB Employee run [the patch build of this
PR](https://github.com/mongodb-labs/ai-ml-pipeline-testing?tab=readme-ov-file#running-a-patch-build-of-a-given-pr)?

### Checklist for Reviewer {@primary_reviewer}

- [ ] Does the title of the PR reference a JIRA Ticket?
- [ ] Do you fully understand the implementation? (Would you be
comfortable explaining how this code works to someone else?)
- [ ] Have you checked for spelling & grammar errors?
- [ ] Is all relevant documentation (README or docstring) updated?

## Focus Areas for Reviewer (optional)

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scrutiny and explain why. -->

Author: caseyclements

.gitignore

@@ -6,6 +6,7 @@ __pycache__
 .env
 .venv*
 .local_atlas_uri
+.ipynb_checkpoints
 docs/langchain_mongodb
 docs/langgraph_checkpoint_mongodb
 docs/index.md