GPU python webdocs have nothing for type 3 transforms

[ahbarnett]

The py doc formatting issues have been fixed by Libin in #768, which also added cufinufft Plan.

However I just noticed that 1d3, 2d3, 3d3 have no docs at all, even though we have type 3 on GPU:

Scroll down this page:
https://finufft.readthedocs.io/en/latest/python_gpu.html