Codesplit example

[runemadsen]

This PR does two things:

1. It rearranges the stylesheets to share some of the files across the PDF and HTML build. This mostly means sharing the font and codesplit code, but I didn't go through all the CSS to delete duplicate code, so that is still needed. Most of the changes are me deleting the duplicated font and CSS files, and moving them into shared fonts and stylesheets folders.
2. It updates the codesplit plugin and runs it on the first code example in the vector chapter. It was good that I did this, as I made quite a few changes to the way the plugin works.

The CSS for the codesplitting still needs a bit of work, as you can see in the PDF rendering. But I would say it's 80% of the way there. Most of the work will be running through the examples and using the new {} attribute command in the code comments. The README will come in handy for this.

Anyway, let me know what you think.

[shiffman]

This is amazing, thank you so much! It's working so well now, I hope to go through it more and make tweaks. Seeing how the CSS can be shared across builds is also super helpful, yay!

[runemadsen]

Great! Let me know if you need help with some of the CSS.