Favor `npx` over global install of Sass migrator

[rdela]

Seems like in Automatic Migration section of Breaking Change: @import and global built-in functions, it would make more sense to recommend npx:

npx sass-migrator module --migrate-deps your-entrypoint.scss

Over a global install of Sass migrator?

sass-site/source/documentation/breaking-changes/import.md

Lines 47 to 50 in c935ee1

	```shellsession
	$ npm install -g sass-migrator
	$ sass-migrator module --migrate-deps your-entrypoint.scss
	```

Let’s make it as easy as possible for folx to get ahead of the deprecated @import breaking change. To me, this means favoring npx over globally installing Sass migrator to automatically update stylesheets to use the module system.

[nex3]

Why is npx particularly easier? The migrator version has no particular need to differ from project to project, and it's a source-level tool so there's not a strong need to share it between multiple developers on a given package. Adding it to package.json only to remove it immediately seems like more of a pain to me.