docs: extract file handling sections into separate tab

[vursen]

The PR reorganizes the Upload component article so that all information on handling uploaded files in Flow, Lit and React is located under a separate tab "File Handling".

Here is an overview of the changes:

• Renamed "Flow Usage" tab to "File Handling".
• Removed the "Setting Maximum Files" section from "File Handling" (already covered on the main tab).
• Combined "Handling Uploaded Files in Flow" from the main tab with "Defining a Receiver Instance" from "File Handling", as they are essentially about the same.
• Renamed "Handling Upload Requests on Client-Side" to "Handling Upload Requests in Lit and React" and moved it from the main tab to "File Handling".