A https://readme.md/ is brilliant for small projects. For very large projects, there are lot of mature tools: mdbook (used for Rust), Sphinx (used for Python) and Scribble (used for Racket).
I don't know of many good options for "slightly bigger than a README" though.
miniblog.
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Automatic sphinx docstrings in Emacs: https://github.com/naiquevin/sphinx-doc.el Nifty, though they can be a little verbose.