General improvments to the documentation - in particular describing the model motivating the procedure

[jokasimr]

• The definitions of some parameters in the workflow are missing. See essreflectometry/amor/typing.py.
• Model motivating the footprint correction coefficient is missing.
• Model motivating the supermirror calibration is missing.

[nvaytet]

Is this about adding the information (mathematical expressions) from the old notebook in the ess repo?

If so, a couple are missing from the list:

• For the resolution function, there are references to an article, but we could also add the math for our docs
• There is also a link to a reference for the conversion to theta, but the mathematical expression could be included

Some of the figures (about angle conventions) in the old notebook are also very useful. We should make sure we don't lose them.

Finally, in the normalization section, there was some discussion about why we compute the mean and not the sum to make the final 1D plot. I remember we talked about this with Drew for a while, and this info should also be preserved.

[jokasimr]

There is some discussion in #44 that might be useful for this issue.