Add some doctest examples for SemigroupK/MonoidK

[ceedubs]

No description provided.

[codecov-io]

Codecov Report

Merging #2077 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2077   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         318      318           
  Lines        5383     5383           
  Branches      207      207           
=======================================
  Hits         5096     5096           
  Misses        287      287

Impacted Files	Coverage Δ
core/src/main/scala/cats/SemigroupK.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/MonoidK.scala	80% <ø> (ø)	⬆️

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[LukaJCB]

Thanks! :)