Fix homepage css

[lbcai]

Fixes #2023

• Adjusted the top header where the sprinter and the star button are
• Made the Contributing section always have 1 column since there are only 2 items there. But if 2 columns are preferred, I'll change that back
• Added a little margin between sections
• Made Social and Learning Resources section swap to 1 column if screen is small enough, just like the other sections
• Adjusted star button for dark mode users. Before:
  
  After:
  
  It's not currently possible to make the iframe background match the color of the dark mode docs. To fix that, we could use this other solution (https://buttons.github.io/) that involves loading a Javascript script, but it seems like this background issue is only present in Firefox and not in Chrome so I imagine it's low priority.

[pushfoo]

TL;DR: Is it possible to split the GitHub stars fix into a separate, smaller PR to merge it in faster?

Visually, this definitely seems to work and I like it. However:

• We're mixing multiple concerns right now
• Adding some clarification comments might help us in the future
• Some of the CSS class names are a little vague, but that might be OK with comments

[pvcraven]

The PR makes things better. Bit more code churn than I'd like.

Looking at it, I think that whole toc-outside-links can be removed. It was originally created when I didn't know how to do links (or was it supported at that point?) in a TOC, so I faked one.

See: this other PR

Think this PR should be merged, then go back and simplify.