Split existing CSS styles into multiple scss partials

[parlough]

This PR doesn't change the styles at all or start to take advantage of sass features for the styles themselves. It just splits the existing styles into multiple scss partials and includes them in the root web/styles/styles.scss file. This will help keep related styles together, as was already attempted with inline comments before some parts of the original file.

[parlough]

@srawlins Any thoughts on this PR? Once this lands I'll start taking advantage of sass features and making some style fixes.

[srawlins]

Hey @parlough sorry for the delay! I'm doing some other stuff until Thursday, so I should be able to give this a review, and manual testing, on Thursday!

[srawlins]

It's funny because I think we really don't use too many styles; hard to check visually but everything looks fine to me. I launched the Dart SDK docs (dart tool/task.dart serve sdk) and everything looks good. Clicked around different types of pages.

[srawlins]

I love the organization so much ❤️ .

After reading the code I realized that I hadn't looked at search or mobile / responsive UI. Both looks great!

[srawlins]

The only thing that caught my eye was how method signatures look in dark mode:

This is the same as dartdoc main branch, so this PR did not change anything. It just struck me as a very low-contrasting grey-on-dark. Something to look into later. Thanks again!

[srawlins]

Merge with main and re-upload?