Link to Foreman docs from WebUI

[maximiliankolb]

          After we have merged https://github.com/theforeman/foreman-documentation/pull/3460, we could also add a link to the official Foreman docs. Thorben and I have already implemented something similar in foreman_remote_execution. I believe there is a shared function that we can use which handles the logic so we'll only have to provide the name of the guide and the anchor.

Originally posted by @maximiliankolb in #89 (comment)