Link the docs site to the shinyreact showcase gallery

[schloerke]

The showcase gallery built by @samuelbharti is worth linking from the docs site — it is the best "what can this actually do" answer we have.

• Live: https://samuelbharti-io-shinyreact-showcase.share.connect.posit.cloud/
• Source: https://github.com/samuelbharti/shinyreact-showcase

Work:

• Add a link from the docs landing page (and READMEs, if we want it there too).

Open question — where should the gallery live?

• Stay under samuelbharti/ and we just link out.
• Transfer to posit-dev/ so it is maintained alongside the packages (with attribution kept).

The hosting URL follows whatever we decide, so answer this before wiring links.