feat: Use UploadHandler in documentation

[caalador]

fixes #4304

[github-actions]

AI Language Review

Here are the issues that require improvements:

1. Upload.java:

   • Line 11: The MultiFileMemoryBuffer import is not used and should be removed for better code cleanliness.
   • Line 12: The InMemoryUploadHandler import is not necessary in all classes and should be removed where it is not used.
   • Comment at line 17 mentions "No explicit Flow API for this at the moment." Ensure this comment remains accurate and provides value. If the situation has changed or the comment adds confusion, it should be updated or removed.
   • For multi-line comments and code blocks identified with // tag::snippet[] and // end::snippet[], ensure that these tags match and are correctly placed to avoid documentation tool issues.

2. UploadFileBuffer.java:

   • The import of InMemoryUploadHandler is not used and should be removed.
   • In comments, ensure that multi-line comments using /* */ are consistent and provide necessary context or instructions.

3. UploadDropLabel.java:

   • Ensure consistency in using icons throughout the project. If using Vaadin components, be consistent with their style and initialization.

Each code file should be reviewed for unused imports, unnecessary comments, and ensure consistency across similar components. If comments and code blocks are intended for documentation processing tools, ensure accurate and matching tags for proper parsing.

[mshabarov]

In addition to other comments, I'd would add to the article a description for UploadEvent, UploadMetadata and
(can be borrowed from downloads docs) also for TransferContext, allowInert, urlPostfix and disableMode.

[mshabarov]

Converted to draft to not being merged before RC/GA

[mshabarov]

Methods getRequestSizeMax, getFileSizeMax and getFileCountMax of UploadHandler should be documented.
Also responseHandled is mentioned in examples, but not described in a sentence.