Add FAQ for downgrade to free after trial

[KiraLempereur-Spacelift]

Added explicit info on how customers are automatically downgraded from the Enterprise trial to the free plan.

Description of the change

Checklist

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Note

Low Risk
Documentation-only change with no impact on runtime behavior or user data.

Overview
Adds a new Billing FAQ entry explaining that Enterprise trial accounts automatically downgrade to the Free plan when the trial ends, with guidance on where to see remaining trial days and what access/limits to expect afterward.

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