Applies to branches 3.x

As a doc owner I want to make sure that the following article is split to separate information related to Custom Limitations:
https://doc.ibexa.co/en/latest/guide/custom_policies/

based on information provided by the engineering team in the following PR.
https://github.com/ezsystems/developer-documentation/pull/1133

For example:

"would be great to add information to the doc about how to create custom limitation e.g. CustomLimitationType.php and CustomLimitation.php classes with sample configuration {{Security\Limitation\CustomLimitationType:
tags:

• {name: ezpublish.limitationType, alias: CustomLimitation}

  "}}

Also, as part of this effort I want to add code samples, as suggested above as well.

Specifically, code samples should be added in the following paragraphs:
"If you want to provide human-readable names of the custom Limitation values, you need to implement [`EzSystems\EzPlatformAdminUi\Limitation\LimitationValueMapperInterface`](https://github.com/ezsystems/ezplatform-admin-ui/blob/master/src/lib/Limitation/LimitationValueMapperInterface.php)."

"To provide support for editing custom policies in the Back Office, you need to implement [`EzSystems\EzPlatformAdminUi\Limitation\LimitationFormMapperInterface`](https://github.com/ezsystems/ezplatform-admin-ui/blob/master/src/lib/Limitation/LimitationFormMapperInterface.php)."