Add doctest example for groupByNel

[ceedubs]

Mostly stolen from the groupBy example on NonEmptyList.

[kailuowang]

This is a handy one. Thanks for adding the doc @ceedubs !

[codecov-io]

Codecov Report

Merging #2225 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2225   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files         333      333           
  Lines        5789     5789           
  Branches      211      211           
=======================================
  Hits         5503     5503           
  Misses        286      286

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/list.scala	100% <ø> (ø)	⬆️

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