Currently, Data Science Notes is built on the Jupyter Book platform. This means that Jupyter Notebooks, plain Markdown files, and several other formats are available to compose content.
The authors recommend so far to compose .md files within a Markdown editor that supports robust markdown syntax including support for LaTeX typesetting. An editor that supports custom HTML blocks as well as other Markdown plugins could make the workflow easier as well. Mark Text is a good choice app and is available for free currently.
The Write Book Content section of the Jupyter Book website can prove quite useful for seeing the syntactical possibilities avaiable to contribute content.
Content should be written in a similar form as other sections utilizing tools such as LaTeX typesetting or highlighted code snippets if necessary.
Please include sources at the bottom your content if applicable!