Add miniGPT-Jax to community Examples.

[yilunyu]

What does this PR do?

Add miniGPT-Jax repository as a community example in Flax documentation.

Fixes #4181 (issue)

Checklist

• [x] This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• [ ] This change is discussed in a Github issue/ discussion (please add a link).
• [ ] The documentation and docstrings adhere to the documentation guidelines.
• [ ] This change includes necessary high-coverage tests. (No quality testing = no merge!)

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