update contribution guide for 2.7/3.0
Fixed some code indentation
Fix indentation of YAML example
Linked the PDO/DBAL Session article from the Doctrine category
replace Symfony2 with Symfony
Update the_controller.rst

Corrected error in the sample code.
Update controllers.rst
Spelling mistake tens to tons
consistency, replace "e-mail" with "email"
consisteny, use "front-end" instead of "frontend" or "front end"
consistency, replace "save-handler" with "save handler"
unify "Symfony Standard Edition" usages
use "stylesheet" instead of "Stylesheet"
"web debug toolbar" instead of "Web Debug Toolbar"
use Boolean instead of boolean
use "object-oriented" instead of "object oriented"