This is official documentation for the OP-TEE project. Before OP-TEE v3.5.0 it used to be spread across all different OP-TEE gits making up the OP-TEE project as well as optee.org. But starting with OP-TEE v3.5.0 we have gathered all documentation at single place (i.e., this git).
Even though GitHub renders *.rst somewhat OK, you are not suppossed to browse the documentation there/here. Instead you should go to optee.readthedocs.io, where you will find the complete documentation rendered using Sphinx.