DOC: ecosystem.rst: Pandas and Ray

[westurner]

What should the ecosystem.rst heading and description for Ray be?

• DOC: Pandas and Ray
  DOC: Pandas and Ray ray-project/ray#1711

https://github.com/pandas-dev/pandas/blob/master/doc/source/ecosystem.rst

[westurner]

https://ray.readthedocs.io/en/latest/pandas_on_ray.html

[westurner]

http://ray.readthedocs.io/en/latest/pandas_on_ray.html

Pandas on Ray has moved to Modin!

Pandas on Ray has moved into the Modin project with the intention of unifying the DataFrame APIs.

Modin project: https://github.com/modin-project/modin

https://rise.cs.berkeley.edu/blog/pandas-on-ray-early-lessons/

• DOC: update URLs & import statement