feat: Add docs for DownloadHandler

[mshabarov]

Adds a documentation for new API for handling downloads in Vaadin 24.8.

Fixes #4303

[github-actions]

AI Language Review

The provided files are new, so I am reviewing only the ".new" versions for clarity, accuracy, and completeness.

1. Dynamic Content.adoc:

   • Under the "Using Custom Servlet and Request Parameters" section, the phrase "You can create a custom servlet which handles 'image' as a relative URL" should be clarified. Consider rephrasing to specify that this involves configuring a new servlet to respond to requests at the "/image" path.
   • Consider specifying that the @WebServlet annotation requires import from javax.servlet.annotation so readers know it's not provided out of the box in some environments.
   • The explanation of SVG XML could be expanded slightly to ensure readers understand they might need to adjust or expand the SVG content as needed for different use cases.
   • In the "Using StreamResource" section, consider more explicitly stating that StreamResource allows integration without managing servlet configurations.

2. Downloads.adoc:

   • When describing "Render Or Download Static Resource," clarify that a preceding slash in the resource name specifies an absolute path, affecting where it will look for the resource.
   • In "Download A File From File System," add that file path specification should be secure and caution against exposing sensitive directories.
   • Under "Download Progress Listeners," it could be beneficial to explicitly state the possible need for thread safety when using certain callbacks, even though Vaadin manages UI.access().
   • For the "Disabled Update Mode" section, the explanation around specifying that it overrides the default behavior of downloads could be elaborated to emphasize implications on security and user experience.
   • In "URL Postfix," the explanation can be expanded to clarify what part of the URL structure it affects and why it might be beneficial or necessary to use in dynamic URL generation scenarios.

The documentation is generally comprehensive and well-structured. Just make sure to clarify the contexts and implications of technical configurations for users who might be new to servlet and Vaadin environments.

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for the new DownloadHandler API in Vaadin 24.8, including multiple usage scenarios and API customization examples.

• Removed the legacy dynamic content documentation.
• Added detailed documentation for DownloadHandler covering classpath resource downloads, file system downloads, dynamic InputStream downloads, progress tracking, and custom download handlers.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
articles/flow/advanced/dynamic-content.adoc	Legacy documentation removed.
articles/flow/advanced/downloads.adoc	New documentation added for the DownloadHandler API.

Comments suppressed due to low confidence (1)

articles/flow/advanced/downloads.adoc:241

• [nitpick] The class name 'LinkWithM5Validation' is ambiguous; consider renaming it to something more descriptive, such as 'LinkWithChecksumValidation'.

private static class LinkWithM5Validation extends Anchor {

[mshabarov]

Converted to draft to not being merged before RC/GA

[mshabarov]

I've added the recently introduced AttachmentType and placed the DownloadEvent and lambda example in the first place in the list, also reordered the helpers a bit.