Add get started and improve help for Gitpod, GDK and review apps

Why is this change being made?

This MR aims to:

• Improve discoverability and guidance for troubleshooting with review tools (especially local GDK and Gitpod).
• Promote Gitpod and review apps’ benefits and improve “getting started” documentation.

See insights from the internal survey done for the FY22-Q3 KR: Improve the quality of design MR reviews.

Author Checklist

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