Added notebook titled "backoff_and_retry_for_LLMs.ipynb" to the /notebooks/community/generative_ai/ directory

[pemujo]

Summary: I added a new notebook titled "backoff_and_retry_for_LLMs.ipynb" to the /notebooks/community/generative_ai/ directory.
This notebook demonstrates how to trigger and handle 429 Quota exceeded errors using backoff and retry strategies with Dynamic shared quota. The notebook includes the accompanying code for an upcoming blog post on the same topic.

PR is for Community Notebooks under the notebooks/community folder:

• [x] This notebook has been added to the CODEOWNERS file under the Community Notebooks section, pointing to the author or the author's team. The change is included on this PR
• [x] Passes all the required formatting and linting checks. You can locally test with these instructions.

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[gericdong]

@pemujo Looks good. Please fix the linter errors