Add guide for creating walkthrough illustrations

[daviddossett]

Now that more extension authors are contributing walkthroughs for their extensions, I think it makes sense to add a guide to our extension docs for creating the artwork with support for theming as we did for the core walkthroughs.

This could include:

• A step-by-step example of creating an illustration for a walkthrough step
• Links + example of how to use the VS Code Color Mapper figma plugin
• Examples from core

cc @digitarald for any other input based on discussions with those adopting the walkthroughs

[digitarald]

@daviddossett thanks for filing! I'd be happy to scaffold a walkthrough guide and you could help to add the illustration section.

[daviddossett]

Even better! That sounds great 👍