GitHub
Taken from the Wikipedia article, GitHub is a “Proprietary software” developer platform that allows developers to create, store, manage, and share their code. It uses Git to provide distributed version control and GitHub itself provides access control, bug tracking, software feature requests, task management, continuous integration, and wikis for every project.
The source code for the documentation you are reading is maintained in a GitHub project. Unless you are logged into the GitHub account that owns that project, you will not be able to see it.