Improve the visibility of the Templating documentation

Template management is part of the Administration Application extension (the url: http://extensions.xwiki.org/xwiki/bin/view/Extension/Administration+Application#HTemplatesCreationandAdministration). It isn't an easy place to find and it is mixed with very technical topics (ConfigurableClass, captchas, ...).

I think this feature deserves a page in the Administrator Guide (http://platform.xwiki.org/xwiki/bin/view/AdminGuide).