Update prepare.md links

What does this Merge Request do?

This MR updates references to prepare.md which now only exists to redirect to index.md 🙂

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.