Docs: IOTask-queue

Since it is such a central concept, the [IOTask-queue](https://github.com/openPMD/openPMD-api/blob/dev/include/openPMD/IO/AbstractIOHandler.hpp#L120) deserves a *conceptual* documentation in the `Development` section of our sphinx manual.

@C0nsultant can you please add a little page on it? Be aware that [breathe](https://github.com/michaeljones/breathe/) allows you do plainly include doxygen doc strings of namespaces, classes, defines, etc. where needed. With that you can avoid writing the same text twice, if e.g. the `openPMD::IOTask` class already has a good doxygen doc.