Adding ShieldGemma 2 notebook to Responsible AI Toolkit docs

[RyanMullins]

Description of the change

Adds a Colab Notebook describing how to use the newly released ShieldGemma 2 model for image content moderation, part of Google's Responsible Generative AI Toolkit.

Motivation

Documents the expanded ShieldGemma family of models supporting text and image content moderation needs of GEnAI application developers.

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.
• 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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[markmcd]

Just a note to make sure this will smoothly publish from GitHub -> Devsite

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PR author updated, per request.