Sphinx is an open-source application development framework that enables developers to quickly create web and mobile applications. It is based on the Python programming language and provides a number of features for developers to easily create complex applications with minimal code. It includes a powerful template engine, an object-relational mapping system, an internationalization system, and a command-line interface. Additionally, Sphinx also provides integration with popular web frameworks such as Django, Flask, and Pyramid. The framework is designed to be lightweight and fast, yet powerful enough to support a wide range of applications. It is easy to use and provides a number of features to help developers quickly develop and deploy their applications.
ReadTheDocs (RTD) is a Sphinx-doc hosting site, not a docsite generator itself and therefore not an alternative. RTD is a great complement to the use of Sphinx.
Good design, logical approach to documentation organization, hosted on GitHub Pages. That's great.