Updating docs to Symfony 4 and Symfony Flex

[javiereguiluz]

This meta-issue keeps track of the docs upgrade to Symfony 4 and Flex:

Directory	Pull Request
assetic/	#8750
best_practices/	#8599
bundles/	#8613, #8617, #8620
components/	#8747
configuration/	#8732
console/	#8748
contributing/
controller/	#8664
cookbook/	(update not necessary)
create_framework/
deployment/	#8721
doctrine/	#8729
email/	#8667, #8745
event_dispatcher/	#8670
form/	#8755
frontend/	#8750
http_cache/	#8682
introduction/	(update not necessary)
logging/	#8683
profiler/	#8685
quick_tour/	(partially) #8781
reference/	#8643
routing/	#8657
security/	#8735
serializer/	#8687
service_container/	#8751
session/	#8689
setup/	#8544
templating/	#8734
testing/	#8705
translation/	#8733
validation/	#8756
workflow/	#8654

[javiereguiluz]

Articles that need important rewordings:

• /contributing/code/reproducer.rst
• /quick_tour/the_architecture.rst
• /introduction/from_flat_php_to_symfony2.rst
• ...

[weaverryan]

See #8798.

Other than the quick tour, do we need to do anything else in this issue? If not, we should close it and open a separate issue for the quick tour.

[javiereguiluz]

Closing as fixed. The rest of things to update are #8884 and #8885.