Add a monthly link check

[IsaacG]

Copied from https://forum.exercism.org/t/link-checker-workflow/86659/ and exercism/jq#397

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[glennj]

Tons of the gawk manual links at gnu.org are really flaky and time out. It might be beneficial to change them to point to archive.org.

[IsaacG]

Tons of the gawk manual links at gnu.org are really flaky and time out. It might be beneficial to change them to point to archive.org.

gnu.org should be an authoritative source and active. I'd be more inclined to ignore them so long as the resource still works. Thoughts on ignoring gnu.org instead?