Add documentation for upload event arguments

[falkoschindler]

Motivation

The Discord user MasterOfNone asked about documentation for the event arguments of ui.upload. So far we only described them in the release notes for version 3.0.

Implementation

This PR doesn't try to demonstrate all properties and methods, but simply lists them with a short description. In combination with other demos this should be sufficient.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests are not necessary.
• Documentation has been added.

[evnchn]

Minimally viable, but I think maybe we can slip in some demo code as well (may be another PR)

I am thinking:

ui.upload(on_upload=lambda e: ui.notify(f'Uploaded {e.file.name} of size {e.file.size()}')).classes('max-w-full')

[evnchn]

I am also considering a simplier solution where we are less likely to run into stale documentation, if we did:

doc.reference(ui.upload.FileUpload, title="Reference for FileUpload")

[falkoschindler]

I am thinking:

ui.upload(on_upload=lambda e: ui.notify(f'Uploaded {e.file.name} of size {e.file.size()}')).classes('max-w-full')

There's another demo using e.file.text(). So we basically cover calling methods on e.file. And I'd rather not make the lambda expression any longer (way beyond the normal window size).

I am also considering a simplier solution where we are less likely to run into stale documentation, if we did:

doc.reference(ui.upload.FileUpload, title="Reference for FileUpload")

Good idea. I added this reference section in 67b2e54.
I'd still keep the section about "Upload event arguments" because most properties and methods are not covered by any demos and the reference section is a bit hard to find.