Add workflow to check markdown files for broken links

[vishnuravi]

Add workflow to check markdown files for broken links

:recycle: Current situation & Problem

@StanfordBDHG and @StanfordSpezi repositories have markdown files with many external links, including Swift Package Index documentation and others. It is cumbersome to manually ensure that all links in markdown files are working and frustrating for users who encounter broken links.

:gear: Release Notes

Adds a workflow that can be used to check a repository's markdown files for broken links using markdown-link-check.

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[PSchmiedmayer]

Thank you @vishnuravi, this looks great and is a great addition to the automated checks 👍