Resolve docs linting error

What does this Merge Request do and why?

Older versions of sed don't behave the same so instead, let's use the version basic cat command to append README.md to the list of .md files to check for broken links.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1422 (closed)