New link checker failing too often with "too many requests"

[tuminoid]

For example, metal3-io/metal3-io.github.io#571 has plenty of failures due "too many requests", or repeated calls leading to 500 error.

Find a way to rate limit the checker, so it doesn't push that many requests. Even GITHUB_TOKEN does not allow this much spam in that short notice.

/kind ci
/triage accepted

[tuminoid]

/assign @peppi-lotta