chore: more fully remove assetgraph-builder and canvas

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🛠 Repo: Unable to install on ARM Mac (node-pre-gyp and gyp failures) #5059
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

This is a superset of #5069 as reverted by #5095. Replaces the existing assetgraph-builder + canvas system in two ways:

• Instead of using assetgraph-builder to optimize the site's assets, resizes larger images to how they're used now and runs npx imagemin-cli on all the .pngs
  • Result: does away with the docs/_dist directory, and instead has the docs/_site be the final output
• Instead of using canvas to determine image positioning in their fixed size parent DOM elements, uses CSS

Long-term I'd want us to move to a more modern docs site builder, with or without a reorganization to multiple pages. But this works in the interim I think.

[voxpelli]

👏👏👏 I like it!

Would want to merge this before continuing with the last parts of #5128, which is moving out the docs from nps

[voxpelli]

Scratch previous comment – but we need to adapt either this one to the other one or the reverse

We should merge this prior to #5071 at least though

[JoshuaKGoldberg]

If you'll pardon my reach, this one makes it annoying for me to review other PRs, so I'm going to sneak it in first. 😄

[coveralls]

coverage: 94.523%. remained the same
when pulling d059d41 on JoshuaKGoldberg:again-remove-canvas
into a8f8c8b on mochajs:main.