improve link-validation action

[jan--f]

I ran the new link-validation action for the first time. We should update the jobs config somewhat to ignore repo-local links, e.g.:

docs/guides/file-sd.md:5: link /docs/prometheus/latest/configuration/configuration/#kubernetes_sd_config, normalized to: /home/runner/work/docs/docs/docs/prometheus/latest/configuration/configuration: file not found

as well as tutorial links to localhost like

docs/guides/multi-target-exporter.md:290: "http://localhost:9115" not accessible even after retry; status code 0: Get "http://localhost:9115": dial tcp [::1]:9115: connect: connection refused