Enhance docs and warnings with distribution compatibility info

[ankitkhushwaha]

Description of the Issue

As mentioned by @dhuppenkothen in comment,

it would be nice addition to add explanation in the docs which methods support which statistical distributions (e.g., Poisson, Gaussian).
This would help users quickly understand which methods are appropriate for their data and reduce confusion.

[Sauravroy34]

I guess it be nice if we add methods to calculate gaussian error as well currently we only calculate for Poisson errors only
https://github.com/StingraySoftware/stingray/blob/main/stingray%2Flightcurve.py#L404-L421

[dhuppenkothen]

You cannot calculate the Gaussian error, that isn't something you can derive directly from the light curve as you do with the Poisson error, but depends on the instrument used and its characteristics, so has to be supplied by the user.