Prepare WordPress Playground publishing

[priethor]

Part of #283.

Cortext should have a public Playground link that installs the same ZIP we publish in GitHub Releases. The demo blueprint can live in this repo, while the official gallery entry should use WordPress Playground's bundled resource flow.

• Add the demo blueprint in the Cortext repo.
• Add a README link for trying Cortext in Playground.
• Document how the demo blueprint is built and verified.
• Add the gallery blueprint files for WordPress/blueprints.
• Open the WordPress/blueprints PR after the first GitHub Release ZIP exists.