Improve the Libraries page

[amirmc]

http://preview.ocaml.org/learn/libraries.html

There's not much on this page so we either need to flesh it out or remove it.

[agarwal]

I feel like it is somewhat helpful for people to know all the different places to search for packages. There are a lot of libraries without an opam package, so you can't purely rely on it.

[amirmc]

Ok. Then we'll need to improve it and link to it from more places in the site.

[agarwal]

I'll add a bit more commentary on the different items.

[Chris00]

See also ocaml/ocaml.org#497 and ocaml/ocaml.org#502