GitBuilding is an OpenSource project for documenting hardware projects with minimal effort, so you can stop writing and GitBuilding. GitBuilding is a python program that works on Windows, Linux, and MacOS. More information on the GitBuilding project, or how to install GitBuilding please see the GitBuilding website
To edit the documentation you do not need to install anything, but you will need to install something to build the final version of the documentation (such as a website). The documentation files can be opened in a plain text editor such as Windows Notepad, Notepad++, gedit, VS Code, etc. GitBuilding also comes with a browser-based editor that displays has a live display of the final HTML documentation.
If you have ever used markdown you will notice that the files you are editing are markdown files. GitBuilding uses an extended markdown syntax (that we call BuildUp). This allows you to keep track of parts in the documentation. More detail on the documentation is available on the GitBuilding website. There is also additional syntax for configuration, and for part libraries.