Add link checking to CI #143

A few recent PRs fixed dead or wrong links in the tutorials (#121, #124, #139). It would be nice to catch these automatically before they land.

Proposal: add a link checker (something like lychee) as a step in the PR check workflow, so broken links get flagged on the PR.

One thing to watch out for: some sites block bots and show up as false failures even though they work fine in a browser. From the recent link sweep, the usual offenders were doi.org (403), unsplash.com (401), and usgs.gov (202). So the checker needs an accept-list for those, or it will be noisy.

Happy to set up the workflow and the config if this sounds useful.