Describe Vaadin Upload component

[glebfox]

Existing page: https://docs.jmix.io/jmix/files/uploading-files.html

Currently, only fileUploadField and fileStorageUploadField are described, but Vaadin Upload field is also available in XML.

Vaadin doc: https://vaadin.com/docs/latest/components/upload

In addition to Vaadin receivers, we implemented FileTemporaryStorageBuffer and MultiFileTemporaryStorageBuffer.

receiver can be set declaratively using either receiverType or receiverFqn attributes:

<upload id="upload1" receiverType="MULTI_FILE_MEMORY_BUFFER"/>
<upload id="upload2" receiverType="MULTI_FILE_TEMPORARY_STORAGE_BUFFER"/>

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Questions can be addressed to @Flaurite