Simplify README

The list of the commands is starting to make less and less sense to me. With all the logic we've introduced, the list does not really work. I created wiki pages for the each of the commands, which is basically an HTML version of the man page. I'm hoping that once the wiki pages are good, this PR can go through.
Do we want to provide the form without spaces here? `git extract` instead of `git-extract`. Sure, the commands are technically implemented as `git-extract` under the hood, but the user sees `git extract`.