Add sample code for getting values in different forms

Problem/Motivation

The docs on the element class are really clear and comprehensive!

It would be nice if they also mentioned ways to get the values from the element.

E.g. if you only have a single child element, you can do this:

    $values = array_column($form_state->getValue('multi_value_element'), 'child_element');

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes