Facilitate serving properly sized images in next-gen formats

[westonruter]

Feature description

One of the most significant performance problems with webpages is heavy images which are sized too large for display (e.g. full size in an image which is better suited for medium) and which are using inefficient formats (e.g. JPEG instead of WebP):

This gets raised in the support forum, as well as via the AMP Page Experience guide. As part of incorporating the PX guide into the plugin (#5679), we should do its same checks for whether modern properly-sized images are being served, and if not, provide guidance on how to do so. One easy remedy is to enable the Site Accelerator image CDN (aka Photon) in Jetpack, or to recommend one of the other reputable image optimization plugins (e.g. Cloudinary).

There could be a Site Health test that links over to a documentation page on amp-wp.org.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This is a natural next step after tests for object caching (#5780) and page caching (#4386).

[westonruter]

Images are some of the absolute worst offenders for frontend performance, so this makes sense to prioritize.

[westonruter]

This makes more sense to be tackled in https://github.com/WordPress/performance