Link checker

[lwasser]

Let's look and fix our CI and to make sure that the link checker skips GitHub references. They tend to be fickle, but we often link to them, causing a huge volume of pings, which can result in a 429 Too Many Requests error.

We might want to consider turning off link checker with htmlproofer (and let it just handle alt tags, etc) and moving to a lychee approach like we use in our website repo

https://github.com/pyOpenSci/pyopensci.github.io/blob/main/.github/workflows/linkcheck.yml

[tkoyama010]

@all-contributors please add @lwasser for ideas

[allcontributors]

@tkoyama010

I've put up a pull request to add @lwasser! 🎉