📦 Add linkcheck action

[TechWiz-3]

In this PR, I have added a workflow which checks links and fails if any of them are broken.

Disclaimer: this action is my own, the repo is here.

Because of the large volume of links, I'm using the 'fast' version/branch which simply requires a GitHub token (with no extra perms) to be stored as a repo secret under the name TOKEN. This is for requests to the GitHub API to avoid rate-limits. More setup info here

You can view the logs of the workflow on my fork here. 2 links were found to be broken.

Let me know if you'd like me to change anything, any feedback is appreciated either way :)