[#2914][WIP] Additions to the new authentication articles

The biggest difference is the extension of the API article to give more
details about the user provider and to talk about how you might store
authentication information in the session.
Update composer installation command

The actual command have a reference to 2.3.* version
adding XML code block
[#3357] Many many changes thanks to @WouterJ and @xabbuh
Merge pull request #3357 from symfony/security-2914-additions

New Security Authentication Additions
Merge pull request #3361 from xabbuh/fix-build-errors-2.4

[Components][DomCrawler] adding XML code block
Merge pull request #3362 from unairoldan/patch-1

Update composer installation command
Merge branch '2.4'
[#3362] Updating composer install version on the master branch