Add import math lib example code

[bertyhell]

I never find how i need to import modules in scss anywhere in the docs

So propose we add it at the top of docs page

Maybe this can be added to other module pages as well in separate PR's

[netlify]

❌ Deploy Preview for sass-lang failed.

Name	Link
🔨 Latest commit	99f4152
🔍 Latest deploy log	https://app.netlify.com/sites/sass-lang/deploys/65004543f19a960008b613c6

[nex3]

I think a better way to do this would be to add appropriate @use rules to each individual code example in the page, so that they're fully runnable in isolation. This also works well with the idea in #779 of linking examples directly to the playground, since otherwise these examples won't run in the playground. I've filed #830 to track that.