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

• 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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