Add help link to three rather important pages

Add help link to three rather important pages

* Specialpages is a useful hub, but not so visited. The help
  page is currently not that good, but is not controversial either.
* Preferences should obviously be better, but may be better than
  nothing. Some explanations and links there are definitely useful.
* Categories don't worry much and there's no doubt they require more
  education to be really understandable and useful for users. They
  are however sort of content pages, if users start thinking they can
  get help about the contents of the categories of specific wikis
  we'll need to remove the link.

Bug: T45591
Change-Id: I7445419864e85685b3ca0cf8333f38b284c71111