Add a user guide page for grid transformations and fft filters

[santisoler]

Description of the suggestion:

After #238 we would need to add a new page to the user guide that covers grid transformations, and optionally another one that covers how we could use the frequency domain filters independently.

Before tackling this issue, I would upload a sample grid of gravity or magnetic data to Ensaio so we can use it in the docs (instead of having to generate one in the same example). I'm more inclined to find an already built grid with open license, but if we cannot find one we would need to pregenerate it.