Improve EntityComboBox doc

[Flaurite]

Description

Existing page: https://docs.jmix.io/jmix/flow-ui/vc/components/entityComboBox.html

• [x] Add an example of using MetaClass together with itemsContainer.
• [x] Add an example of configuring FetchCallback - XML and Java.
• [x] Add an example of custom Renderer, see https://demo.jmix.io/ui-samples/sample/combobox-custom-renderer
• [x] Add attributes: ariaLabel, ariaLabelLabelledBy, css, focusShortcut, overlayClass
• [x] Remove attributes: invalid, requiredIndicatorVisible
• [x] Add elements: itemsQuery, prefix
• [x] To discuss. I think we can add to notes that beside EntityPicker you can configure FetchCallback for lazy loading.