feat: Use UploadHandler in documentation

[caalador]

fixes #4304

[github-actions]

AI Language Review

The changes in the source code primarily replace the previous file upload approach using MultiFileMemoryBuffer with a new method, employing InMemoryUploadHandler from UploadHandler. Key areas for review include:

1. UploadAllFiles.java:

   • The snippet comment indicates // tag::snippet[] without providing surrounding context or description. Provide clear context for the snippet for better understanding.
   • Retain consistency in comments when referring to hidden lines for training models or code samples that should not be a permanent part of the code.

2. UploadAutoUploadDisabled.java:

   • Ensure consistency in using InMemoryUploadHandler for better code organization and readability.

3. UploadBasic.java:

   • You might need to provide appropriate context for the metadata fields (fileName, mimeType, contentLength) used within the lambda block for InMemoryUploadHandler. This would aid any developer in understanding the significance of the metadata.

4. UploadDragAndDrop.java:

   • Ensure consistency in using InMemoryUploadHandler over multiple instances to aid clarity in handling drag-and-drop functionalities.

5. UploadDropLabel.java:

   • Ensure the Upload component metadata or additional handlers do not throw unexpected errors by testing scenarios where metadata information might be incomplete or malformed.

6. UploadFileBuffer.java:

   • Transition from using separate FileBuffer and MultiFileBuffer to FileUploadHandler approaches. Ensure the generic approach via UploadHandler.toFile(...) accommodates both single and multi-file scenarios seamlessly.

In terms of documentation, the transition to using InMemoryUploadHandler and FileUploadHandler needs to reflect changes clearly across documentation examples, accommodating for both single and multi-file uploads seamlessly. This would potentially impact various examples detailed under sections such as "Handling Uploaded Files in Flow", "File Handling", and "Upload Progress Tracking". Ensure any mention of previous handler methods (MultiFileMemoryBuffer, FileBuffer) are updated wherever appropriate to align with code changes to UploadHandler implementations.

[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.