Archive old files

Here's a preliminary go at archiving old files.

1. I may have gone a bit overboard. Just let me know.
2. I didn't archive `v8.html` because Google shows links to it.
3. I used `old/` instead of `archive/` since I felt the former was less ambiguous, but it doesn't matter much to me what you call the directory.
3. I added `old/` to the Jekyll exclude directory so that it doesn't try to build from those files.