docs: compare shinyreact with shiny.react and friends; explain reactive_output

[schloerke]

Summary

Adds a website article, articles/comparison.qmd, contrasting shinyreact with Appsilon's shiny.react (the one-dot name collision), reactR/reactable, shiny.fluent/shiny.blueprint, shiny-bindings, and the whole-frontend templates. It has three parts: a side-by-side table against shiny.react; a "what travels the wire: markup or data" section that writes the same filter+table+caption task with renderReact(), renderReactable(), and reactive_output; and a "can they be combined" section documenting how to host a shiny.react tree and a reactable inside a shinyreact client via ShinyOutput.

Expands the Getting Started page (index.qmd) with "The power of reactive_output" and a one-output-drives-three-components example in Python, R, and ui.tsx. Links the README's related-work list to the article and adds reactable and shiny.blueprint.

The two interop paths were verified in a browser (chromote) against an R page_react() server with the current bundle: a shiny.fluent tree rendered through renderReact() re-renders on input change and its *.shinyInput components set Shiny inputs; a reactable renders, re-renders, and reports row selection to getReactableState(). Two gotchas are documented: renderReact() is not a shiny.render.function, so reactDependency() and shinyReactDependency() must be added to the page by hand; reactable needs the data-reactable-output attribute on the ShinyOutput element or its state never reaches the server.

Verification

cd pkg-py/docs && quarto render index.qmd && quarto render articles/comparison.qmd

Both render. The py/index.qmd link warning is pre-existing (quartodoc output not generated locally). No code changes; docs only.