feat: Add docs for DownloadHandler (#4309)

* feat: Add docs for DownloadHandler

fixes #4303

* Update articles/flow/advanced/downloads.adoc

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix review comments part 1

* fix review comments part 2

* fix review comments part 3

* fix lists

* add more advanced examples, simplify existing

* remove CSS and JS

* Update articles/flow/advanced/downloads.adoc

Co-authored-by: caalador <mikael.grankvist@vaadin.com>

* update example

* update example

* add attachment type

* place the DownloadEvent and lambda example first

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: caalador <mikael.grankvist@vaadin.com>

Author: 3 people