link: add checker for external links

[McShelby]

Linking external resources can become stale over time without recognition.

Add a configurable link checker for external resources for link, image, include and openapi

[McShelby]

While still a great idea and on my list, this has to wait a bit. Hugo recently made backwards incompatible changes in 0.141.0 to the API and I don't want to raise the minimum version, yet.

Perparations where already done in #971 and #1002 to call the check in the required locations. Currently the check simply does nothing.