Although InDesign and Scibus can be used to finish off a product. Before that, but after you've finished the content. You may want to get your writing formatted into a formal writing style using something like markdown or for more detailed cases, LaTeX.
This is where Overleaf comes in. Its an online editor for LaTex. Allowing you to work with other people on your project to make a quality well structured rpg.
Overleaf is an online LaTeX and Rich Text collaborative writing and publishing tool that makes the whole process of writing, editing and publishing scientific documents much quicker and easier.