publish: pass `cname` to cut over from the interim github.io URL to the custom domain

[mmcky]

Surfaced by the QuantEcon/actions v0.8.0 validation study.

Context

lecture-dp is the one composite-migrated repo still serving on the interim github.io URL. Its publish step (.github/workflows/publish.yml:69) calls quantecon/actions/publish-gh-pages@v0.8.0 without a cname, so it's not exercising the custom-domain path in production. This is a deliberate interim state, not a defect.

When ready

Pass the custom domain to publish-gh-pages to cut over:

      - uses: quantecon/actions/publish-gh-pages@v0.8.0
        with:
          cname: dp.quantecon.org   # confirm the intended domain
          # ...existing inputs

Confirm the exact domain — the other lecture sites follow <name>.quantecon.org (e.g. jax.quantecon.org, python-advanced.quantecon.org).

Context: QuantEcon/lectures#11 (v0.8.0 standardization).