Refactor Furo to use Sass nesting

[Eric-Arellano]

Sass allows nesting:

When writing HTML you’ve probably noticed that it has a clear nested and visual hierarchy. CSS, on the other hand, doesn’t.

Sass will let you nest your CSS selectors in a way that follows the same visual hierarchy of your HTML.

E.g.

nav {
  ul {
    margin: 0;
    padding: 0;
    list-style: none;
  }

  li { display: inline-block; }
}

vs.

nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
nav li {
  display: inline-block;
}

This cleans up some of our hard-to-read selectors.

[Eric-Arellano]

Reviewers: tip that this is easier to review using GitHub's split view mode

[frankharkins]

Looks good, just one question but it's non-blocking