Migrate marketplace assets to a public repo + GitHub Pages

Repo: Jumoo/Public

Public: new repo to host static assets/marketplace files, so they stop polluting the .github org profile repo
Public: migrated Umbraco Marketplace listing JSON + screenshots out of Jumoo/.github (originals left in place for now, pending reference cleanup)
Public: set up GitHub Pages (plain branch deploy, no build step) on a custom domain, assets.jumoo.co.uk
Public: added a landing page and README
Website: Updated the marketplace files and assets to the new urls.