Add automated link checker for cg-site

[markdboyd]

To prevent broken links from appearing in our public documentation, we should add automated link checking to our CI for this repo.

Tasks:

• Choose an automated link checker and Github action
• Add a Github workflow for link checking to this repo
• Test Github workflow for link checking to make sure it works correctly
• Submit PR to fix broken links
  • #2107

[markdboyd]

For now, I'm going with the lychee link checker tool and action:

https://github.com/lycheeverse/lychee
https://github.com/lycheeverse/lychee-action

And I have a WIP PR adding a workflow for the Lychee action:

#2104