Introduction to images should also cover lazy-loading

[zcorpan]

https://html.spec.whatwg.org/multipage/images.html#introduction-3 has an introduction to responsive image features.

Then there's a subsection "Adaptive images" (which predated the above introduction), covering the same thing but with more focus on how to specify dimensions and which image to use as "the fallback image".

I think the Adaptive images section should either be removed or folded in to the earlier introduction.

Also, it would be nice to introduce other image-related features, at least lazy-loading. sizes=auto (#8008) has a dependency on loading=lazy, but it's a bit out of the blue to mention lazy-loading in the introduction about responsive images.

If we add multiple subsections to the image introduction, I think we should call the current introduction "Responsive images" (rather than adaptive images).

[zcorpan]

Specifying dimensions could maybe be a topic on its own, as the width and height attributes provide the aspect-ratio even when CSS overrides one of the dimensions to auto, and the width and height attributes on <source> could use an introduction.