Added notes specifying the propagation behaviour for kernel.view and kernel.exception.
Add missing info about kernel.request event.
Changed phrasing to explain the effects off propagation stopping.
Fix incorrect event name.
Updated installation instructions to use the new Symfony Installer
Removing some installation instructions
update contribution guide for 2.7/3.0
Restored the original section title
Add  missing autoload include in basic application example
Minor grammar fix
Fixed section headers
update ordered list syntax
Rewritten from scratch the chapter about installing Symfony
Re-added a wrongly deleted link reference
Fixed some code indentation
Fix indentation of YAML example
Linked the PDO/DBAL Session article from the Doctrine category
minor #4649 Linked the PDO/DBAL Session article from the Doctrine category (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #4649).

Discussion
----------

Linked the PDO/DBAL Session article from the Doctrine category

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4416

Commits
-------

cb4784a Linked the PDO/DBAL Session article from the Doctrine category
replace Symfony2 with Symfony
minor #4652 replace Symfony2 with Symfony (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4652).

Discussion
----------

replace Symfony2 with Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

224c283 replace Symfony2 with Symfony