Updated Jenkins documentation

Using SSH URLs instead of Anonymous HTTP in template, a workaround for Jenkins problem.
Added missing MediaWiki URL to template.
Moved MediaWiki username and password from general Jenkins configuration to jobs. That allows us to use different user for every job.
Documented all exising jobs.

Bug: 55729
Bug: 55728
Change-Id: I48e974a544a010913a1bc9b79f8c1a97fd23ed80