Documentation for the new Download API

[mshabarov]

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Document the new API for downloads from server that replaces StreamResource API.

AC:

• Old content in https://vaadin.com/docs/latest/flow/advanced/dynamic-content is removed
• New article https://vaadin.com/docs/latest/flow/advanced/downloads replaces it
• New article describes how to use four new ready-made DownloadHandlers as well as a custom implementation and including prefix, inert, disabled customisations.
• New article gives examples for Anchor, Image and also mentions IFrame, HtmlObject, SvgIcon, Avatar, AvatartGroup and MessageListItem.
• New article describes how to listen for download progress with the listener and shortcuts.

Could use similar examples from vaadin/platform#7469.

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