mdoc task should fail on warnings of broken links

[bplommer]

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#2955 fixes some broken links in the docs - mdoc warns about this, but it's a runtime warning log rather than a compiler warning so fatal warnings don't catch it. I'm not sure whether failing on warnings can be configured here or if this needs an issue in mdoc.

[armanbilge]

~Note there are two steps on site generation: mdoc and Laika, and AFAIK Laika fails on broken links. Have you found a broken link that slipped through?~

Edit: woops, wrong project :) Usually see Ben over one http4s! But, this is good reason to consider switching to sbt-typelevel-site plugin ;)