Improve FileUploadField doc

[Flaurite]

Description

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

1. Add information that the field can work without data binding. And add an example.
2. Add information that field stores uploaded file in-memory, i.e. no file created in the file system.

3. Add attributes:
   • css
4. Remove attributes:
   • invalid
   • requiredIndicatorVisible