Make it clearer to users how to go from prototype plan to free plan in billing

[andrewnhem]

User on support wasn't sure how to go from prototype to free plan. Checking on billing docs, it would be worth an update to represent the latest, including screenshots to map how to actually downgrade plan.

There are some cases where this is difficult to do, e.g. prototype fleets with tons of devices on them, but for people with few devices who upgraded then want to downgrade, it could be clearer.

https://www.balena.io/docs/learn/manage/billing/#changing-your-billing-plan