Expand pricing and billing documentation

[pelcasandra]

Summary

• expand Pricing, Billing & Plans from three guides to ten bilingual guides organized into four navigation groups
• document the highest-only billing model, performance fees, fair-use messaging, Meta fees, SMS pricing, usage, invoices, payment methods, plan changes, and troubleshooting
• remove the duplicated plan matrix so current prices and plan contents remain sourced from the public pricing page
• clarify that Replenishment Driver estimates cadence from AI, business history, and customer history, while an explicit usage_duration_days value is an optional highest-priority override

Why

This makes billing information easier to find and less likely to become stale, while resolving customer confusion that an explicit replenishment duration was required before launch.

Validation

• git diff --check origin/main...HEAD
• yarn build with the repository Ruby 3.3.6
• production security-header verification
• verified the English and Spanish billing navigation and replenishment pages on the local Jekyll server

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