Add document for GridExtensions

[nandin-borjigin]

Docs for Toolkit PR #4474

What changes to the docs does this PR provide?

PR Checklist

Please check if your PR fulfills the following requirements:

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

[opbld30]

Docs Build status updates of commit 0e83b9f:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/extensions/GridExtensions.md	:white_check_mark:Succeeded	View
docs/resources/images/Extensions/GridExtensions.dynamic-layout.gif	:white_check_mark:Succeeded	View
docs/toc.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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