feat: add Github action to check for broken links

[vijay-jaisankar]

Ref: https://github.com/gaurav-nelson/github-action-markdown-link-check

This would a good addition to the project as it

• Reduces manual work performed while checking links
• Can detect links that stop working after they have been pushed to the repo

[EmmaDawsonDev]

Oh this would be brilliant! I'll definitely look into adding it.