Create garthdb/design-data-spec repo + initial scaffolding

Part of #1006.

Create the personal repo `garthdb/design-data-spec`, add a README explaining this is a Bikeshed prototype for evaluation (linking back to adobe/spectrum-design-data), and configure GitHub Pages to publish from Actions.

Start by reviewing the parent issue #1006 and the spectrum-design-data repository context. Create garthdb/design-data-spec with the requested README describing the Bikeshed prototype and linking to adobe/spectrum-design-data, then configure GitHub Pages publishing from Actions. Done means the repository exists, the README is present, and the Pages workflow publishes successfully.

Written by the indexing model from the issue text.