Create documentation for Wikis

Dev: https://dev.gitlab.org/gitlab/gitlabhq/issues/2477

We don't have documentation about wikis yet.

https://twitter.com/suthecoder/status/618772541142867968

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• Main directory: user/project/wiki/
• Main document that will list all functions user/project/wiki.md or user/project/wiki/introduction.md (to be discussed)
• What to cover (maybe split into separate documents):
  • First time creating the Home page
  • Create a new wiki page via the online interface
  • Delete a wiki page via the online interface
  • View a list of all created pages
  • View the history of a wiki page
  • Edit a page
  • Add and edit wiki pages locally
  • Use different markup languages
  • Disable wiki from your project

Points worth mentioning:

• Wiki is Git-based
• Every change should have a commit message otherwise one is created for you.
• How the Home page is different than the others (cannot be absent)