new way of creating extensions for twig
Merge pull request #2457 from ricardclau/update_twig_extension_creation

[Cookbook][Templating] New way of creating extensions for twig
Add a new section on front controllers and the AppKernel
Some updates
Markup fixes and small changes
Even more GH feedback, some additional notes and typos
GH feedback
Not mention the Debug Component until it's ready
Mentioning the Debug component again as it has just been merged into master.
[#2465] Proofreading new environment, front controller, kernel doc
Merge remote-tracking branch 'origin/2.1' into 2.1
Merge branch '2.0' into 2.1
Merge branch '2.1' into 2.2
Merge branch '2.2'
[#2465] Fixing tiny typo
Merge branch '2.0' into 2.1
Merge branch '2.1' into 2.2
Merge branch '2.2'
[#1465] Re-adding debug component note