RelengAPI documentation is processed with Sphinx.
As you can see from the documentation you're reading, the Releng API documentation is divided into a few high-level chapters, including this one, "Development". Documentation for blueprints will follow this same pattern.
To add a new chapter (which should only happen rarely!), add a new element to DOCS_DIRS in setup.py.
Every project that implements a blueprint can include documentation.
This documentation should be placed in the project's docs, directory, in docs/{chapter}/{project-name}/index.rst.
The project-name here should correspond to the lowercase version of the name in the project's setup.py.
Be very careful not to add files in any intermediate directories, as those will override files in the base Releng API project.
The index.rst file can contain a toctree pointing to other files, or just the desired content.
For example, usage documentation for a "bumper" project would be in docs/usage/relengapi-bumper/index.rst.
See Documentation for information on documenting REST API endpoints and types.
Before it can be seen, documentation must be "built".
If your project is installed in a virtualenv with setup.py develop or the equivalent pip install -e, then you must use the --develop (-d) argument to relengapi build-docs.
This option re-copies the documentation from the source tree to the location where Sphinx expects to find it (under sys.prefix).