docs: point live-demo link at namespace Pages URL

Andrew Dunnrequested to merge
docs/demo-url-namespace into main

The demo project (gitlab-com/public-sector/manifold-demo) had its Pages unique-domain routing disabled, so the demo now serves at the path-based namespace URL https://gitlab-com.gitlab.io/public-sector/manifold-demo/ instead of the random unique-domain host. Update the README live-demo link to match. Doc-only.

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