docs: add detectors to the API reference #155

the API reference (docs/api.md) doesn't have a detectors section at all, so none of the detector classes (CameraCCD, CameraEMCCD, CameraCMOS, and now PMT / HyD from #147) show up on https://talleylambert.com/microsim/

probably just needs a ## Detectors section with ::: microsim.schema.detectors and a short intro... but worth checking how it renders (the lib presets with their big QE arrays, and the private _Detector base where most of the docstrings now live).