I'll get around to making a new mission template and example for TethysAPI soon. Obviously OPUPatch uses it extensively, but that's not going to be a good reference for coding missions. I did start adding Doxygen comments, mostly under the API folder, but I honestly haven't tried actually using Doxygen to generate docs from it yet.
The beauty of TethysAPI being header-only is you don't need to link any libs or anything at all. It's easier to integrate than the old SDK, just drop it in a folder called Tethys.