[docs] Add ray backend in the docs of parallel_backend.

[AmeerHajAli]

Added documentation for the ray implementation of joblib backend.
@GaelVaroquaux

[codecov]

Codecov Report

Merging #1110 into master will decrease coverage by 14.59%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1110       +/-   ##
===========================================
- Coverage   94.52%   79.92%   -14.60%     
===========================================
  Files          47       47               
  Lines        6955     6779      -176     
===========================================
- Hits         6574     5418     -1156     
- Misses        381     1361      +980     

Impacted Files	Coverage Δ
joblib/parallel.py	96.45% <ø> (ø)
joblib/test/test_dask.py	3.80% <0.00%> (-95.07%)	⬇️
joblib/_dask.py	24.00% <0.00%> (-70.53%)	⬇️
joblib/test/test_memmapping.py	37.07% <0.00%> (-62.17%)	⬇️
joblib/format_stack.py	58.33% <0.00%> (-41.67%)	⬇️
joblib/backports.py	44.73% <0.00%> (-39.48%)	⬇️
joblib/my_exceptions.py	64.28% <0.00%> (-35.72%)	⬇️
joblib/_memmapping_reducer.py	61.74% <0.00%> (-34.87%)	⬇️
joblib/executor.py	76.19% <0.00%> (-23.81%)	⬇️
joblib/pool.py	72.13% <0.00%> (-15.68%)	⬇️
... and 22 more

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[ogrisel]

Thanks!