Implement "category" property to organize UI Examples better

MR is ready 🎉 I am using the following code in the ui-examples-overview-page.html.twig template.

{# Build grouped array #}
{% set grouped_examples = {} %}

{% for example in examples %}
  {% set group_name = example.group|default('Other') %}

  {% set current_group = grouped_examples[group_name]|default([]) %}

  {% set grouped_examples = grouped_examples|merge({
    (group_name): current_group|merge([example])
  }) %}
{% endfor %}


{# Sort keys alphabetically #}
{% set group_names = grouped_examples|keys|sort %}

{# Rebuild sorted array, skipping "Other" for now #}
{% set sorted_groups = {} %}

{% for name in group_names %}
  {% if name != 'Other' %}
    {% set sorted_groups = sorted_groups|merge({
      (name): grouped_examples[name]
    }) %}
  {% endif %}
{% endfor %}

{# Append "Other" at the end if it exists #}
{% if grouped_examples['Other'] is defined %}
  {% set sorted_groups = sorted_groups|merge({
    'Other': grouped_examples['Other']
  }) %}
{% endif %}

{% set grouped_examples = sorted_groups %}

{% for group_name, examples in grouped_examples %}
  <h2>{{ group_name }}</h2>

  {% for example in examples %}
    {{ example.label }}
    {{ example.description }}
  {% endfor %}
{% endfor %}

This is ready for review again 🎉 In order to test this properly the requested ordering from #3574277: Order UI examples with the same logic as the other UI Suite modules lives in this MR as well now.