make documentation more accessible

[soenkehahn]

I realized that the haddock rendering of the tutorial module (Generics.Eot.Tutorial, for example here: https://www.stackage.org/haddock/lts-11.3/generics-eot-0.2.1.2/Generics-Eot-Tutorial.html) ended up without the tutorial comments. Therefore I thought it was actually harmful to include that module in the package -- because it might prevent people from finding the real tutorial (here: http://generics-eot.readthedocs.io/en/stable/tutorial.html). Therefore I removed that module from the exposed modules of the package.

This PR also includes various other tweaks that makes the documentation hopefully more discoverable.

CC: @gwils

[soenkehahn]

I still have to remove the markdown-unlit dependency from the library section of the package.

[gwils]

:+1: Looks good to me, once the markdown-unlit dependency is dropped.

[soenkehahn]

@gwils: Thanks a lot for reviewing. I removed the markdown-unlit dependency and also your changes involving build-tool-depends, since they are not needed anymore to use generics-eot as a dependency with cabal new-build. Will merge as soon as CI goes green.