DOC: UploadViewDefinition

The newly introduced UploadViewDefinition allows:

• The (DAM) upload field to be configured using custom factoryClass. If no factory class is configured, the default (DAM) upload field will be rendered.
• The (DAM) upload field to be marked as required and to display a validation error for an empty field.

Java definition class: https://git.magnolia-cms.com/projects/PLATFORM/repos/ui/browse/magnolia-ui-framework/src/main/java/info/magnolia/ui/editor/UploadViewDefinition.java