Add detail to Pry docs

What does this Merge Request do?

Add some more detail on how to use Pry with GDK.

Clarification on how Thin replaces Puma, as asked in Slack

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.