This is the source of the knowledge base in Tabletop Simulator. It uses a modified version of Material-Design for MKDocs.
The .md files in the /docs folder are written in Markdown, which is an easy-to-use markup language to add formatting to text. Additionally, some custom CSS is used, as well as a handful of custom images. When making changes, it is possible to live-preview them as you go if you have set up the local files for mkdocs + material design.
Alternatively, you can make modifications to individual pages then submit them for review. The developers will always be the ones to build and publish the site anyway, all you will do is modify the contents of this Git.
If you choose to install MKDocs so you can live-preview your changes, you may do so by following these instructions.
Otherwise, you will not need to install anything to edit the text files.