feat: Use UploadHandler in documentation

[caalador]

fixes #4304

[github-actions]

AI Language Review

• In the .java source files, the changes to use InMemoryUploadHandler and UploadHandler seem to be consistent across the modified examples. However, ensure the examples clearly explain when and why one should choose between different types of UploadHandler implementations, as this choice may not be immediately obvious to new users.
• Each example initializes InMemoryUploadHandler at the beginning of each class. Consider adding brief comments explaining the purpose of these handlers for clarity, particularly for less experienced developers.
• In several .java files, such as UploadFileBuffer, methods use formatting like System.out.printf() for console output. Ensure consistency in using logging instead of System.out.print* methods where appropriate. This follows best practices for production-level reliability and flexibility.
• In the AsciiDoc documentation, ensure all transformations of the Receiver component to UploadHandler have clear migration instructions included for developers who might be using previous patterns.
• When explaining UploadHandler implementations, consider elaborating on usage boundaries such as memory limitations in InMemoryUploadHandler compared to FileUploadHandler, which uses the file system.
• Ensure consistency in the usage of inline code blocks in the documentation (e.g., always using backticks ... for method names), as inconsistencies can diminish readability.
• In the AsciiDoc sections transitioning from Receiver to UploadHandler, specify which methods or practices are obsolete or outdated to prevent confusion among users who might refer to older documentation.

[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.