* igorw/docs:
  [docs] note that twig is not included
  [docs] add Extension suffix to extension titles
  [docs] some documentation for the UrlGeneratorExtension
  [docs] better examples for the extensions chapter
  [docs] remove reference to symfony_bridges in TwigExtension docs
  [docs] move extension documentation to sub-directory
  [docs] remove _exts
  [docs] more on routes and twig
  [docs] initial documentation draft