add a link check to our docs building GH action

[falkben]

As an example:

• https://github.com/karllark/measure_extinction/actions/runs/913582579

[falkben]

I don't remember what I was suggesting we do here

[mdecleir]

I think I suggested this. It is a way to test if links in the documentation are still working. The links get tested when the docs are built as part of the Github actions. When you were converting my pdf with instructions into the documentation, you discovered some of the links were broken. So this testing is a way to avoid this, without having to check them manually. This is not high priority, but might be worth putting in eventually if it is not too much effort...

[falkben]

Thanks! Now I remember