Improve documentation

[CFGrote]

I found the following issues with the documentation;

Quickstart expectations: I would expect an example simulation as part of a quickstart, not just instructions for installation.

Redundant sections: Introduction/Quickstart and Users/Installation are more or less identical, remove one of them

installation uses python setup.py which is kind of old fashioned. pip install -e . is the way to go.

Calculators: Really just one Calculator? Can we have more?

PhenomSourceCalculator:

• should describe that this is a XFEL pulse and how it is modeled
• being picky: Why do we need all this wpg stuff, can't we put all that in some common namespace or (better) provide wrappers for common plotting tasks?
• While at it, should also show how to use the sampling check in wpg.
• also, since backengine installation only comes next, wpg might not be available, yet

Backengine installation

install from git without cloning:

pip install git+https://github.com/...@simex

mention that mpi is a requirement

WPG:

why install from juncheng repo, not central WPG?

Usage

Add introductory explanation of what is being done, what are the steps

• add comments to code
• turn doi into html
• explain the img_gaussian parameters
• What does "Larger slop and intercept gets larger noise" mean?