Documentation for the new Upload API

[mshabarov]

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Document the new API for uploads that replaces Receiver old API.

AC:

• https://vaadin.com/docs/latest/flow/advanced/stream-resources is removed
• https://vaadin.com/docs/latest/components/upload is updated to use UploadHandler for existing examples
• https://vaadin.com/docs/latest/components/upload describes three ready-made UploadHandler implementations as well as a custom implementation and including prefix, inert, disabled customisations.
• https://vaadin.com/docs/latest/components/upload#listeners is updated to use TransferProgressListener

Could use similar examples as in vaadin/platform#7469.

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