I use Google Drive (or Google Docs or Google One, depending on what they call it this week) for basically everything. It’s free, it auto-saves, it’s the same no matter what computer I’m on, and it generally works great. This extends to tabletop RPG design. You could use expensive desktop programs like Microsoft Word, or Adobe Indesign… but they don’t get you as much as you’d think, at least when you’re starting out. Google Docs works great for writing game rules. It’s basically just like writing in Microsoft Word, except it saves to the cloud, so you never have to worry about losing your work.