This is an Opensource project. That means that it is free of charge, and that you can see, modify and suggest changes to the source code. The whole project stack is opensource, so you can contribute to this projectitself, the documentation or even the blog.
Think that this is a huge project that has been initially developed by a single person, and that requires a lot of time. So probably it will have some rough edges. But you can help to improve it!
Did you find something wrong? Something misleading or confusing? Or just want to add more content to the documentation?
In order to make it easier to contribute, each page of the documentation contains a pencil link, that sends you to the github page of the documentation to modify the file itself.
That link sends you to the file just for viewing, but github provides another pencil button that when pressed will create a fork of the documentation project so you can modify the file and later create a PR with your suggestions.
This project uses jekyll for its documentation. Jekyll is a blogging engine that works out of the box on github. It also allows to be used as a plain CMS allowing to create custom sites like this documentation.