Updated extension UX guidelines

[daviddossett]

We've just released the first wave of updates to the extension UX guidelines. Check them out here. Namely we…

• 🆕 Overhauled the overview page to better step through the entire UI
• 📃 Added missing pages (e.g. Sidebars, Panel, Walkthrough)
• 🎨 Updated all visuals
• 🔗 Added links to guides, samples, and API references to each page

Just the start—more to come!