Overhaul or remove "About PISA"

[thehrh]

Apart from the issues that

• supposedly, "the analysis guide" explains the difference between "parameterized-Monte Carlo (MC) stages and reweighted-MC stages",
• and that the readme refers to the ezFIT documentation and suggests using it as an alternative analysis framework,

"About PISA" looks more like an attempt to outline the basics of frequentist parameter estimation than a description of PISA. Should it be removed (also given https://github.com/icecube/pisa/blob/master/README.md)?

Keep in mind, however, the general convention (https://github.com/icecube/pisa/blob/master/pisa/general_conventions.md) of "one README.md file within each directory to document the directory's raison d'être, or what is common to all of the things contained in the directory."