Add association rule mining section to the docs

[zStupan]

It would be nice to have a section in the user docs that would serve as an introduction to ARM and numerical ARM. In it we could also explain the idea and inner workings of this approach, i.e. a short summary of your articles on DE-ARM @firefly-cpp.

[firefly-cpp]

Yes. I totally agree with you. We can prepare a very short summary of main mechanisms.