Nth PR on develop branch for easybuild-docs, for N=5

still bits and pieces to improve, but ok to receive the first round of feedback
@boegel you'll have to settle on something that you call "EB usage". for me that is either
 * install extra piece of software (implying things already exist like toolchain, and that the new piece doesn't need a new toolchain)
 * install via robot (and do not care about the runtime, it just works)

installing GCC is not impressive, but `eb --robot WRF` is. but this documentation is not about impressing people, it'sa bout documentingh how EB is used. (imho)