Fix custom limitation example

[mnocon]

This PR fixes the following error:

 ------ --------------------------------------------------------------------------------------------------------------------------------------------------------------- 
  Line   back_office/limitation/src/Security/Limitation/Mapper/CustomLimitationValueMapper.php                                                                          
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  :13    Return type bool of method App\Security\Limitation\Mapper\CustomLimitationValueMapper::mapLimitationValue() is not covariant with return type array of method  
         Ibexa\AdminUi\Limitation\LimitationValueMapperInterface::mapLimitationValue().                                                                                 
 ------ -----------------------------------------------------------------------------------------

The interface is typehinted to return array, not bool as in our example:
https://github.com/ibexa/admin-ui/blob/main/src/lib/Limitation/LimitationValueMapperInterface.php#L23

Other things done:

1. Extracted CustomController to separate file and fixed PHPStan issues (except one, which I've put into the baseline).
   The last commit contains changes coming from Rector - added the interface introduced in IBX-10170: Introduced AccessCheckController contract user#111
2. Ibexified the examples - changed occuranges of ez to ibexa

Notes:

1. It's working!

2) The Notification errors reported by PHPStan are fixed in https://github.com//pull/2911 (on 4.6 first), I will merge them to 5.0 and remove the baseline changes once that PR is approved.

[github-actions]

Preview of modified files

Preview of modified Markdown:

• docs/permissions/custom_policies.md