Add walkthroughs for getting started

To help new users with onboarding, a couple of walkthroughs to guide them through the most commonly used features of the extension, and how to set up Regal, would be helpful.

Reference:
https://code.visualstudio.com/api/ux-guidelines/walkthroughs
https://code.visualstudio.com/api/references/contribution-points#contributes.walkthroughs