Improve stylesheet and HTML

[stalecu]

I did some improvements to both the HTML and the stylesheet as is, including:

1. Making the site more HTML5-friendly (I added some helpful metadata in )
2. Turned it into an one-column setup (I tried doing it with a 2x2 that turns into a stack, but I didn't see it fit)
3. Added normalize.css so I get the same baseline styling across all browsers
4. Made the font responsive (so it should adapt across all screen sizes)
5. Fixed the inconsistencies in the links
6. Made a light/dark mode so it respects your preferences
7. Changed the font stack so it more closely resembles Helvetica/Arial (and chooses nicer fonts before that) and same thing for the monospace font stack
8. Other various improvements that I forgot about.

Here are some screenshots:

I wasn't aware of #2 before making this, maybe a combination of our ideas would work best here.