Add URL checker functionality to GHA

[jaclyn-taroni]

Broken out from #12

If we want to link to code where we perform certain actions we are documenting (e.g., generating a splici index [#12], run alevin-fry, generate a QC report), it is easy to envision a scenario where the link to the code is updated and the link in the docs is broken.

We will want to implement some sort of URL checker which ensures that the links we provide in the docs aren't broken. I know @davidsmejia did something similar for the links between refine.bio examples and the refine.bio frontend.