Fix links to `Documentation` in README

Created by: erinaldi

Hello,

I noticed that the links at the beginning of the README file are outdated.

• Citing seems to have moved to cite at https://www.netket.org/cite/
• Documentation is now at https://netket.readthedocs.io/en/latest/index.html
• Tutorials have also been moved to the readthedocs page https://netket.readthedocs.io/en/latest/tutorials/