Docs: Revive useful links section. Update URLs.

[vpavkin]

Also minor templating fixes

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

Fixes #860.

[ceedubs]

Nice! Thank you, @vpavkin. 👍

[codecov-io]

Current coverage is 89.28%

Merging #861 into master will not affect coverage as of f1d1369

@@            master    #861   diff @@
======================================
  Files          168     168       
  Stmts         2323    2323       
  Branches        75      75       
  Methods          0       0       
======================================
  Hit           2074    2074       
  Partial          0       0       
  Missed         249     249       

Review entire Coverage Diff as of f1d1369

Powered by Codecov. Updated on successful CI builds.

[fthomas]

👍 Awesome, thanks @vpavkin!