[RDY] Add Doxyfile and doxygen-style comments to some functions in os/os.h.

This PR
* adds a Doxyfile
* adds Doxygen-style comments to the functions of `os/fs.c`

Watching how the Doxygen documentation grows could be a nice motivation to add proper comments to the functions.

Can we somehow auto-generate the Doxygen documentation in a public place after each merge into `master`? Or at least periodically like every day or so?