Hello! For an overview of Squire and instructions on how to use it, see the repo's readme document. More in-depth documentation can be found here.
Each page linked below contains documentation for one of the built-in plugins.
Something important to note is that each plugin defines two separate set of options: config options and render options. Config options are hard-coded values that are defined in config files and remain consistent throughout the entire build process. Render options are those that are passed in to the plugin each time content is rendered, and so are generally specific to that particular content that is being rendered.
Unless you are interfacing with a plugin programmatically, you should only be concerned with config options.