docs: add migration guide for svgo v4

[SethFalco]

The SVGO v4.0.0 release should be coming out soon, so we need to formalize all of the breaking changes into a document to help users with migrations from v3 to v4.

My hope was that there'd be so little that the release notes could adequately cover it, however there are quite a few breaking changes now, and the release notes are far too verbose to be practical to apply all at once.

The migration guide is a concise version of the release notes, with breaking changes only, and favors instructions over explanations. Users are of course welcome to refer to the release notes, pull request, or create an issue to seek more information.

Documentation

• Adds a "Migration" section with the first migration page for v3 to v4. I'll iterate on this over time until SVGO v4 has actually been released.
• Renames the removeScriptElement page to removeScripts.
  • We've already got a PR in svg/svgo.dev to add a redirect.
• Drop the Exports section under the Node.js and Browser pages. These are already being maintained in JSDoc and types. Rather than maintaining it in yet another place, I would prefer for users to use their intellisense. Alternatively, we could later export our API to web-based documentation. Anything but maintaining it manually.

Chores

• Use a more concise syntax for the minium Node.js version.

Related

• Pairs with chores: updates for svgo v4.0.0 migration documentation svgo.dev#28