Adding Documentation Section focused on underlying stats without code

[kcormi]

This is (the start of) an attempt to help make more clear to users the underlying model, statistical tests etc... being used by combine.

These pages pages are designed to give users concise but thorough and precise references on the details of what is being done.

The existing documentation includes much of this material spread throughout. But it might be helpful to users to have more complete explanations in one easy to find spot with reminders and references back to that material in other parts of the documentation which are focused around how to run procedures and commands.

Open to suggestions/comments at all levels (overall structure, content, flow, choice of notation etc. ).

For those not familiar with setting up the documentation locally to have a look, please see the instructions in the contributing.md document from #838 (you can see it here: https://github.com/kcormi/HiggsAnalysis-CombinedLimit/blob/contributing/contributing.md). A page which should be identical to the one here has also been put up at: https://kcormi.github.io/HiggsAnalysis-CombinedLimit/ -- the new pages are the ones under the 'what combine does' tab.

[kcormi]

I've left this open for an unreasonably long time for no good reason. I just gave it another check, and despite what I'm sure are many flaws, I am happy enough with it to merge it and make it public. Unless there are any loud complaints soon, I will go ahead with the merge.

Closer to the time of releasing the paper, I will go through and try to harmonize some notation etc.

[kcormi]

Thanks, good points Nick. I changed those cases, I also tried to update the text to match this primary/auxiliary wording better and found some other instances throughout where I made the notation and wording more consistent with what's in the paper.