Add examples to Functor and verify with docstyle

[YuvalItzchakov]

This PR implements #2479 for the Functor typeclass.

Would love feedback on the examples, if anything more complex is needed.

[kailuowang]

Nice. Nit: the indentation changed (from scaladoc convention to javadoc)

[YuvalItzchakov]

@kailuowang Sorry about that, I assumed that we'd have a default scalafmt conf :)

[codecov-io]

Codecov Report

Merging #2483 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2483      +/-   ##
==========================================
- Coverage   95.38%   95.35%   -0.04%     
==========================================
  Files         357      357              
  Lines        6517     6517              
  Branches      279      282       +3     
==========================================
- Hits         6216     6214       -2     
- Misses        301      303       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/Functor.scala	100% <ø> (ø)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...rc/main/scala/cats/kernel/instances/function.scala	96.96% <0%> (-3.04%)	⬇️

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[YuvalItzchakov]

@kailuowang LMK if there are any remaining issues with the indentation.

[kailuowang]

Thanks so much!

[LukaJCB]

Thanks Yuval! :)