Scrollviewer expression animations

[Sergio0694]

Docs for CommunityToolkit/WindowsCommunityToolkit#3220

What changes to the docs does this PR provide?

Adds the docs for the new expression animation APIs added in the PR mentioned above.

PR Checklist

Please check if your PR fulfills the following requirements:

• Correctly picked the right branch to base the change off (master for new features, live for typos/improvements)
• For new pages, used the provided template
• For new features, added an entry in the Table of Contents
• Ran against a spell and grammar checker
• Contains NO breaking changes

[opbld32]

Docs Build status updates of commit ddae46d:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensions/ScrollViewerExtensions.md	✅Succeeded	View
docs/resources/images/Extensions/ScrollViewerExpressionAnimation.gif	✅Succeeded	View

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