Add Documentation section

[ghost]

Advertise the openSUSE documentation better, therefore it would be good to have a prominent link to doc.opensuse.org. Maybe Documentation could even be its own section on the landing page itself, with links to the wiki (maybe with some portals like Documentation and SBP as deep links) and aforementioned doc.opensuse.org page.

This was originally brought up on the os-doc ml: http://lists.opensuse.org/opensuse-doc/2017-06/msg00000.html