docs: clarify ngrok instructions around domains

[joelhans]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Tweaking the language around using ngrok domains—there's the free dev domain that comes with every account (your-domain.ngrok-free.app), but also opportunities for people to create unique/custom/branded ones as well.