The Github Pro Documentation Page
Go To assets/ and you should find files known as tab1, tab2 and so on. These Tabs are the website pages! to create a new page, make a folder and call it tab<numberofpage> and inside create a .md file and name it tab<number>_stuff_here and whatever you right will be whats on the page!
Create a config.yml file inside of the tab file and write link: <nameOfUrl>/!
NOTE: You can add more pages to your site by creating more new files. Each file will be available on your site in the same directory structure as your publishing source. For example, if the publishing source for your project site is the gh-pages branch, and you create a new file called /about/contact-us.md in the tab file branch, the file will be available at https://<user>.github.io/<repository>/about/contact-us.html.
To customize your page even more! Check out the Assets Guide