Add more code examples to the FormBuilder documentation

[BurntimeX]

The FormBuilder documentation is currently very abstract and contains few code examples of how to use it for certain things.

The blog article about the introduction of the FormBuilder, for example, contains some examples that could be included in the documentation: https://www.woltlab.com/article/209-new-features-for-developers-in-woltlab-suite-5-2/

The following sections lack examples:

• [x] Overview https://docs.woltlab.com/6.1/php/api/form_builder/overview/
• [x] Form builder usage in dialogs (aka Psr15DialogForm); can replace the section about DialogFormDocument
• [x] Form data handler: https://docs.woltlab.com/6.1/php/api/form_builder/validation_data/#form-data
• [x] Several form fields: https://docs.woltlab.com/6.1/php/api/form_builder/form_fields/
• [x] (including AclFormField)
• [x] Form structure: https://docs.woltlab.com/6.1/php/api/form_builder/structure/#imaximumlengthformfield

[Hanashi]

@BurntimeX please also add an example for usage of AclFormField. Currently it's really not clear for me how to use this FormField and how to handle this data.