Make edit page for contributing more apparent

Discussion https://gitlab.slack.com/archives/C16HYA2P5/p1515689603000805

Right now the link to edit a doc page and thus contribute to it is right after the content which is not that apparent.

An idea is to put it in the footer like we do with about.gitlab.com, but my concern is that we are used to having the source link in the bottom from about.gitlab.com, and not everybody is accustomed to it. Not sure know how many people scroll all the way down the footer to see the link.

Let's find a better place for it.