Stale dp.quantecon.org references in _config.yml and linkcheck.yml (that domain now serves the DP books)

[mmcky]

Summary

lecture-dp's build configuration and link-check workflow reference https://dp.quantecon.org/, but that domain is the landing page for the Dynamic Programming books (book-dp1 / book-dp2, served from book-dp-public) — not this lecture series. This repository actually publishes to https://quantecon.github.io/lecture-dp/ (GitHub Pages, with no custom domain configured — the Pages cname is null). As a result, several generated URLs and the link checker point at the wrong site.

To be clear about what is not a bug: the in-lecture citations to the DP book — e.g. [DP1](https://dp.quantecon.org) in ifp_opi.md, mccall_model.md, and os_stochastic.md — are correct and should stay; they intentionally link to the book. This issue is only about the config and CI references listed below.

Stale references

File	Line	Setting / use	Problem
lectures/_config.yml	28	baseurl: https://dp.quantecon.org/	Canonical URLs, sitemap, and OpenGraph tags resolve to the books' site
lectures/_config.yml	120	tojupyter_urlpath: "https://dp.quantecon.org/"	"Download notebook" links in generated .ipynb point to the books' domain
lectures/_config.yml	121	tojupyter_image_urlpath: "https://dp.quantecon.org/_static/"	Notebook image paths point to the books' domain
.github/workflows/linkcheck.yml	20, 22, 31	Mirrors https://dp.quantecon.org/ as the site to check	The link checker validates the books' site, so this repo's own links are effectively never checked

Why it matters

• The published site (quantecon.github.io/lecture-dp/) advertises canonical / OpenGraph URLs on a domain it does not control and that now hosts different content.
• Notebook downloads generated by sphinx-tojupyter reference a path under the books' domain, so those links are wrong for readers of this series.
• linkcheck.yml has been mirroring and checking the books' site rather than lecture-dp, so the series' internal links are effectively unchecked.

Suggested fix

Decide the canonical domain for this series and align all four references to it. Current planning points to dynamic-programming.quantecon.org as the intended home; until that domain is live, the correct value is the actual Pages URL https://quantecon.github.io/lecture-dp/. If dynamic-programming.quantecon.org is adopted, it should also be set as the Pages custom domain (with the matching CNAME).

Found while planning the QuantEcon lectures monorepo, during a sweep to harmonise series URLs. Happy to open a PR for whichever target domain you prefer.