Reorganize and add demo html/css

The questionable thing I did here was to change part of the directory structure, from (`src_html`, `src_css`, `src_js`) to (`src/html`, `src/css`, `src/js`). I did this as an alternative to prefixing `src_` to all the URLs in the HTML file. I think this is the right solution, since those are the URLs we want to end up with in the HTML.

Actually, I think eventually we'll want to use absolute paths everywhere (e.g., `/css/global.css`), because then the paths resolve the same no matter what the URL of the page is. On the other hand, maybe we won't have any need to split the HTML files into subdirectories and so absolute paths won't be useful after all.

I'd appreciate other opinions on this. It's probably not worth spending a lot of time on this issue right now; we can fix it later if need be.