minor #7280 [Security][Guard] Added a picture for Guard Authenticator (a-ast)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #7280).

Discussion
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[Security][Guard] Added a picture for Guard Authenticator

This illustration shows how GuardAuthenticationListener calls methods of Guard Authenticator.

Commits
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ed0e6da Added the picture that shows how GuardAuthenticationListener calls Authentication Guard methods.

Author: xabbuh

_images/security/authentication-guard-methods.png

@@ -426,6 +426,11 @@ Each authenticator needs the following methods:
     after a successful authentication to create and return the token
     for the user, who was supplied as the first argument.
 
+The picture below shows how Symfony calls Guard Authenticator methods:
+
+.. image:: /_images/security/authentication-guard-methods.png
+   :align: center
+
 .. _guard-customize-error:
 
 Customizing Error Messages