Improve KeyValueContainer documentation

[Flaurite]

Environment

• Documentation version: 7.2
• Part of the documentation: Manual
• Existing page: gui_keyvalue_containers.html

Description of the bug or enhancement

Support forum: thread;

I guess, we need to add some examples of using KeyValueContainer:

• how to use it
• add example or at least mention about how to edit such entity;

Also we need to add a link to the sampler demo: datagrid-keyvalue