Straight to user index file:
Change <Directory "/Library/WebServer/Documents"> to <Directory "/Users/whatever/Sites"> ("whatever" should be replaced by your short name.)
Different users to different pages:
If it's not going to be systematic then you'll need one line for each user. One way to do it is to use alias, for example, to alias cars and Joe from your example.
You could also create some rewrite rules and let mod_rewrite take care of it.
(Check the apache doc files shipped with OSX for syntax.)