Clarify project overview

[mdtanker]

Page that contains the error or needs to be improved (paste link):

https://www.fatiando.org/harmonica/latest/overview.html

Description of the problem/suggestion:

@LL-Geo and I discussed the project descriptions on the Harmonica website. We noted that it states Harmonica will not implement "Generic grid processing methods (like horizontal derivatives and FFT). We'll rely on Verde, xrft and xarray for those."

Since these were included in PR #299, the docs should be updated.

• [ ] Remove text from README.md
• [ ] Remove text from overview.rst