Automated Link/URL checker

[dhmlau]

Suggestion

I'd like to propose to use GitHub Action to check whether links referenced in our docs exist. Possible criteria:

• check on each pull request on loopback-next - if the changes in PR causes any broken links
  • if it has broken link, mark CI as failed
• run a cron job (maybe in loopback.io?) - run daily or periodically to make sure we don't have broken links
  • if it has broken link, open a PR automatically
• able to specify an ignore list - we have quite a lot of references on http://localhost:3000/explorer or similar URL for the API explorer.
• (optional) able to specify the directory to be scanned - supposedly we only want to scan /docs folder

Some investigation

I did some investigation and found https://github.com/marketplace/actions/link-checker might be the closest of what we want, but it doesn't have the "ignore list" capability.

Note: I wasn't able to get it to show as error in the checks yet at the time of opening this issue, it might be because I don't have the right configuration.

[achrinza]

Created #6208 to demonstrate a working (?) example.