Animation frames dev edition is web-developer-unfriendly

[domenic]

https://html.spec.whatwg.org/dev/imagebitmap-and-animations.html#animation-frames

This is entirely implementation details, and is missing the nice "domintro" method descriptions.

Work here should probably wait until after #3677 lands. Then, it would consist of:

• Adding an intro sentence or two to the section. Be sure to mention that it's both Window and DedicatedWorkerGlobalScope.
• Hiding all the current stuff with w-nodev
• Adding a .domintro block explaining the methods for web developers
• Ensuring there's a couple examples (although ideally Worker.requestAnimationFrame spec #3677 should include at least some).