Update shadcn-ui.mdx instructions

[cyupa]

Add missing checkpoint to set up environment variables otherwise the examples do not work.

🔎 Previews:

What does this solve?

• Being a new Clerk user and diving into implementation for my specific use case drove me down a path where I had an incomplete setup.
• This should avoid users having a custom Elements setup and failing to redirect from sign in to sign up and vice versa.

What changed?

• Added an additional step towards having an appropriate setup.
• Fixes second bullet point.

Checklist

• [x ] I have clicked on "Files changed" and performed a thorough self-review
• [x ] All existing checks pass