[WIP] - feat(CI): add a GitHub Action to check Markdown links

[jbampton]

• Run only on pull request
• Checks for dead links in Markdown files
• First run will check all files and then we will have a list of dead links
• Then we can set it up to only run on files added in the PR which will be a lot quicker
• Fix link in README

[jbampton]

Looks like we are getting rate limited by GitHub with the Status: 429 on github.com links. I think this won't happen on normal PRs when we just check modified files and not all files at once.

In ./docs/assets-css.md it's finding a false positive in a code block.

[mtlynch]

I'm a bit reluctant to approve this because this kind of check is very noisy. I have a similar check on my blog, and it's constantly blocking my PRs because a link died on a post from 3 years ago. I'm willing to put up with it because I don't want dead links, but it's definitely a maintenance burden. If we can restrict it to only changed files, that helps, but it's still adding work. Can we get it down to only touched lines in a PR?

I unfortunately don't have the spare cycles to help with this repo any longer, and it doesn't seem like anyone is actively maintaining it. Having a check that fails frequently is going to increase the maintenance load when the project is under-resourced as it is.

Alternative suggestion: we update the README to explain how people can check for dead links themselves instead of baking it into the build.

Looks like we are getting rate limited by GitHub with the Status: 429 on github.com links. I think this won't happen on normal PRs when we just check modified files and not all files at once.

I'd imagine that Github's 429 is per machine or per IP block rather than per repo. I suspect a lot of repos have checks like this and get rate limited, so I think they're blocking nodes in the Github Actions pool of servers rather than our repo in particular.

[hjvedvik]

@jbampton Thanks for the PR. I agree with @mtlynch here. I think we can run the markdown-link-check command manually to fix dead links in all files in another PR, so you can probably make this PR only check for links in the changed files.