You can customize the domain name of your GitHub Pages site with Flybook.
It's quite simple just add the CNAME file into your document root directory look like below
$ ls /path/to/project/docs
docs
|____advanced
| \____theme.md
|____basic
| \____getting-started.md
| \____install.md
|____examples
| \____syntax-highlight.md
|____CNAME <---- add this file
|____readme.md
And you want to know about CNAME in detail. Please see the references link