Improve directory structure to show in descending order.

[MichaelDimmitt]

No new features added with this pull request.

Simply Renamed the directories 1-9 to 01-09.
And updated all of the areas that had a reference to those file-paths.

Namely, index.html and header.html.

This will improve the look of the project on Github.
It will also improve the look of the project when some one has cloned down the project for personal use.

Cheers. 🙂

[MichaelDimmitt]

@redblobgames, here is a screenshot of the improved look when using 01-09 directory names.

[redblobgames]

@MichaelDimmitt I'm no longer active on this project. You can ask @samuelgoto, but I think it's likely that this will wait until someone is actively working on the project. My guess is that the current directory structure will go away at some point, as last year's work wasn't purely chapter-based, and the 4th edition of the book will be changing the chapter structure anyway.