Add markdown link validation workflows

[adhilto]

💡 Summary

Add GitHub workflows to validate that the links in our markdown files are not broken. We might want a few different workflows:

• One that runs regularly (maybe nightly) and checks the links across the all markdown files
• One that triggers when a markdown file is changed and validates just that file
• One to be triggered just before a release that validates all links

Motivation and context

We keep finding broken links. Content owners can change their URLs at any time and we need a mechanism for fast detection.

Implementation notes

There are already tools that have been built for this kind of thing. One that I know of: https://github.com/gaurav-nelson/github-action-markdown-link-check

Acceptance criteria

• workflows have been added