Documentation style

[otizonaizit]

From @guillermoaguilar :

Now about the documentation, https://psignifit.readthedocs.io/en/latest/?badge=latest it is complete and 100% usable, but there were some concerns raised by @otizonaizit last time, that the language or structure might not be "standard" or at the same level of other software packages.

As I counterargument, I say that the package is very specific, it does 1 thing: psychometric function estimation. In that sense it is different from numpy or scipy, which are much more general and thus the documentation should fit many more needs. That's why I think a 'tutorial' style is ok. Still the language
at times is mixed (e.g. "we", "you need to"), and this should be made uniform.