feat: FREE trial messaging, remove re-seeding, update to us-east-1

[chrisns]

Summary

• Remove all cost references and emphasize AWS is providing scenarios FREE via NDX:Try programme
• Create prominent free-trial-banner component with clear "Try at No Cost" messaging
• Add cross-linking between related scenarios for better discoverability
• Remove non-functional sample data re-seeding functionality (simplified to static "Ready" state)
• Update all region references from eu-west-2 (London) to us-east-1 (N. Virginia)

Changes

FREE Trial Messaging

• Removed estimatedCost, maximumCost, costBreakdown from all 6 scenarios in scenarios.yaml
• Created free-trial-banner.njk component with prominent "AWS is providing this evaluation completely FREE" messaging
• Updated homepage hero and trust indicators to emphasize FREE access
• Updated exploration YAML files to show "FREE as part of NDX:Try programme"

Cross-Linking

• Added relatedScenarios arrays to each scenario for automatic cross-linking
• Added getRelatedScenarios filter to eleventy.config.js
• Related scenarios now appear at the bottom of each scenario page

Re-Seeding Removal

• Simplified sample-data-status.njk to show static "Ready" status
• Removed manual seeding instructions from all walkthrough index pages
• Removed API calls and complex JavaScript from sample data component

Region Updates

• Changed all eu-west-2 references to us-east-1
• Updated data residency from "UK (London)" to "US (N. Virginia)"

Test plan

• Build passes (npm run build)
• All scenario pages display FREE badge instead of cost
• Free trial banner appears on scenario pages
• Related scenarios section shows cross-links
• Sample data shows static "Ready" status
• All CloudFormation links point to us-east-1

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