Docs: Add ci-stage for finding dead markdown-links

**Why is this needed**: There are a couple of broken links in markdown-files not below docs/**. See e.g. #92102 #91940 which motivates this request. **What would you like to be added**: A ci-step checking markdown files for dead links. While we have doc-validator for markdowns below docs/** we do not have something in place for e.g. files below contribute/**. doc-validator does not seem to be a good fit for this task. I am evaluating linkspector right now (https://github.com/UmbrellaDocs/action-linkspector ; https://github.com/rwielk/grafana/actions/runs/10463355714/job/28975225176?pr=1) but perhaps there's already a good solution in place I missed. **Who is this feature for?** All contributors to markdown-files not below docs/** @hairyhenderson jfyi