Ensure that local doc references inside documents also exist in the hierarchy.md

[codeallthethingz]

Your proposal or request for the feature

The github_readme_sync tool does check the following broken links:

• Ensures the hierarchy.md references exist on the file system.
• Ensures that references inside a document exist on the file system.

But it does not check (and should)

• that references inside a document exist in the hierarchy.md

Alternatives

n/a

Additional context

This was brought to light when this PR was merged #87

and those missing links were then added in this PR #111