Added a markdown link checker

[mbernier]

This will allow all the links in the markdown text to be checked before a merge or commit.
It can run as a pre-commit hook locally and as a PR merge check.

This should:

1. Run the script inside a containerized GitHub Actions environment.
2. Checks for broken links in .md files.
3. Fails the PR if broken links are found.

Example output:

If a link is broken in README.md:

❌ Error: Broken links found in README.md:
  - [Docs](https://example.com/invalid-page)

To set this up as a GitHub status check on PR merge:

Add a GitHub Status Check
If you want GitHub to block merging until the check passes:

1. Go to Repository Settings → Branch Protection Rules.
2. Add "Markdown Link Check" as a required check.