Add SLES to OS list in Installation page
[Kinda spinned off from #1074 (comment 27579894)]
From 9.1 onwards, we are providing EE packages for SLES - SUSE Linux Enterprise Server - (Related issue - gitlab-org/omnibus-gitlab#1730 (closed) ). So, SLES has to be added to https://about.gitlab.com/installation/ page where we list other OSs/packages. The thing to note is, we are not providing a CE package for it.
/cc @psimyn (Because you recently did the revamp of that page), @luke and @eliran.mesika