Documentation for Modules, Themes, and Layouts contributed by the community.
Most contributed projects (modules, themes, and layout templates) have documentation in the README.md file that is included with the project. Some also have documentation in the Wiki page associated with the Github repository (which will be linked from the README file).
Some contrib projects have additional documentation availble. This could be because they are part of a larger ecosystem of related projects, or just have higer level of complexity.
Please see the list of projects at the bottom of this page for documentation about those specifically.