Get the 2.3 branch back on track

This is the first PR to get the French documentation back on track for easy contributions.

The 2.3 branch couldn't be merged to master as it should be, and as there wasn't any 2.4+ specific feature documented in the current master branch, I simply merged the master branch to 2.3 and resolved conflicts.

So from now with this PR, the 2.3 branch is up-to-date and can be merged back to master.

Any new PR that is not specific to a future version of Symfony should be made on the 2.3 branch (2.2 one when this one is back on track as well) and then merged back to master (by merging 2.3 to master).