Is there a style guide or convention for documents that exist as pure .txt files? I’m looking for the accepted convention to specify titles & headings, lists & dot points, and so on, but without the bold & italics formatting available in conventional word processors.
There’s markdown
This is the right answer. People even write entire books in markdown nowadays (with the proper headings), and you can easily export it to epub, html, whatever you need.
While I prefer reStructuredText, I accepted that Markdown has won this “format war.” It’s used on every platform, and by every company I worked for. OP should use that.