Add Contravariant documentation page

[vpavkin]

Preview http://vpavkin.github.io/cats/tut/contravariant.html

[vpavkin]

Appreciate any input/corrections :)

[ceedubs]

This should switch from non to typelevel. We probably have the same mistake in lots of docs. We could probably just give relative paths here and use site-wide config for the base source URL.

[vpavkin]

Strange, but currently useful links section, with those source links is hidden...
I think this issue and all links should be fixed in another pr. I'll take care of that.

[ceedubs]

I made a few minor comments, but this is great. Thanks, @vpavkin!

[codecov-io]

Current coverage is 89.27%

Merging #859 into master will not affect coverage as of 92ab1fe

@@            master    #859   diff @@
======================================
  Files          168     168       
  Stmts         2322    2322       
  Branches        75      75       
  Methods          0       0       
======================================
  Hit           2073    2073       
  Partial          0       0       
  Missed         249     249       

Review entire Coverage Diff as of 92ab1fe

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[vpavkin]

@ceedubs I'm not a native speaker, thank you for covering my back here! :)
Fixed the minors, will open a separate pr with sources links fixed

[ceedubs]

👍

[non]

Thanks so much! 👍