This documentation is using the Sphinx generator to create the whole documentation and are building automatically using the GiHub Actions.
The custom theme is the official Qiskit theme for Ecosystem project, do not hesitate to go check the documentation in order to know how to custimize it further.
The documentation is structured following the Diátaxis Framework. This framework is for technical documentation that divides it into 4 categories according to the users needs. For further information you can check this page.