Replaced the link to obsolete Pyfft with the link to Reikna.

Reikna is stable enough now to serve as a replacement. Or, alternatively, you can close this PR and remove this section altogether; I do not want people to get the wrong idea about Pyfft status.

If this section stays in docs, perhaps other people would want to add links to their packages as well.

(Yep, this is the exact duplicate of PyCUDA PR)