Add yarn in gdk set up steps for docs

What does this Merge Request do?

I was following the guide and installing node dependencies seems to be missing in the step by step guide

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.