minor #20254 [Doctrine][Security] Remove an old article about registration forms (javiereguiluz)

This PR was squashed before being merged into the 7.1 branch.

Discussion
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[Doctrine][Security] Remove an old article about registration forms

This is another old article soft-deleted a while ago and that we can delete for real in 7.x branches.

Commits
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299e6f5 [Doctrine][Security] Remove an old article about registration forms

Author: javiereguiluz

_build/redirection_map

@@ -569,3 +569,4 @@
 /frontend/encore/server-data /frontend/server-data
 /components/string /string
 /testing/http_authentication /testing#testing_logging_in_users
+/doctrine/registration_form /security#security-make-registration-form

doctrine.rst

@@ -1103,7 +1103,6 @@ Learn more
 
     doctrine/associations
     doctrine/events
-    doctrine/registration_form
     doctrine/custom_dql_functions
     doctrine/dbal
     doctrine/multiple_entity_managers

doctrine/registration_form.rst

@@ -449,6 +449,8 @@ the database::
     Doctrine repository class related to the user class must implement the
     :class:`Symfony\\Component\\Security\\Core\\User\\PasswordUpgraderInterface`.
 
+.. _security-make-registration-form:
+
 .. tip::
 
     The ``make:registration-form`` maker command can help you set-up the