update and simplify installation documentation

[maurov]

fixes #333

@newville I have tried to simplify and clarify the installation page. Unfortunately, it seems that users do not like to understand what is behind the scenes, they just want to install something working, so I have tried to pop out the minimal procedures to get a working (full) Larch installation.

[newville]

@maurov Thanks for that, it looks very helpful! Would it be reasonable to reverse the order to make go from novice to expert:
1. install from binaries
2. use GetLarch.sh/.bat
3. install into your own Anaconda (or other Python?) environment.

That is, for sure we want to make sure that option 3 works and clearly state that it is a normal thing to want to do, and we want to make sure that can work. But if that's listed first, more novice users (less likely to read to the bottom?) might think they have to try that.

Do you agree?

[maurov]

I have chosen this order on purpose. Most of the users are familiar with Anaconda and most probably have already this distribution installed. If not, the install scripts seem to nicely work on Linux/Mac and Windows. The binaries work on Windows and Mac, but fail on Linux.

This said, feel free to change the order, I am fine with that.

[newville]

@maurov I'll merge and then rearrange the order a bit.

I think "most users are familiar with Anaconda" must depend greatly on the facility and area of study. Less than half of the beamline scientists at my beamline are familiar with Anaconda, let alone the users (and I've been trying for years!, they all now know that Python is a thing!). Maybe 1/4 of my users would otherwise have Python installed. That matches my experience with users at training courses and support questions.

But also: I totally get that Anaconda instructions are important.