Add URL checker functionality to GHA

_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.

Start by reviewing issue #12 and the documentation links it discusses, then inspect how GitHub Actions is used in the repository. Define the URL-checking approach and its scope before implementing it. Done means the workflow detects broken links to documented code and reports them.

Written by the indexing model from the issue text.