Add ShieldGemma notebooks

[RyanMullins]

Description of the change

Adds Colab notebooks demonstrating ShieldGemma's use in KerasNLP and Hugging Face Transformers.

Motivation

Adds the Colab notebooks that document example usage of ShieldGemma. See launch announcement: https://developers.googleblog.com/en/smaller-safer-more-transparent-advancing-responsible-ai-with-gemma/

Type of change

Choose one: Documentation

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• [ x My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.

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[RyanMullins]

FYI: The linter check is going to fails because it's finding instance of "we" in a Python string. The uses of "we" in these notebooks is required per the safety policy definitions used by this model.