Improve visibility of extension guidelines #5207

Based on the number of extensions that don't (yet!) align to our extensions guidelines, I think it would be worth exploring how to better surface this material. As of today, the guidelines are somewhat hidden at the bottom of the "References" section, which makes them feel like a bit of an afterthought.

To do

• Lift extension guidelines into a top-level API section
• Move images to UX guidelines section and update routes
• Add overview page to section
• Redirect from old references URL to new location
• Update ToC and Sitemap
• Add new topics to doc feedback table