docs: cycles costs reference page

[raymondk]

Summary

• Replaces the stub with a comprehensive reference page for cycles costs on ICP
• Covers cycle units and XDR pricing peg (with link to live IMF exchange rate)
• Full cost table for 13-node and 34-node (fiduciary) subnets: canister creation, compute allocation, update execution, per-instruction fees, Xnet calls, ingress messaging, and storage — all USD columns scoped to May 2025 rate
• HTTPS outcall cost formula with variable components
• Execution cost formula explaining how base fee + per-instruction fee combine
• Storage reservation mechanism (subnet usage thresholds, reserved cycles)
• Resource limits table (instruction limits, memory limits, message size limits)
• Special features section covering HTTPS outcalls and EVM RPC with TODO for threshold signing and Bitcoin API costs (no verified figures in .sources/)
• Cloud pricing comparison flagged as TODO (requested in content brief, no upstream source found)

Sync recommendation

informed by dfinity/portal — docs/building-apps/essentials/gas-cost.mdx, docs/references/cycles-cost-formulas.mdx, docs/building-apps/canister-management/resource-limits.mdx

[raymondk]

Feedback addressed:

• Scoped all USD columns to "~USD (May 2025)" header label; added explicit note above Cost table that USD values are approximate and cycle counts should be used for precise budgeting
• Added <!-- Needs human verification --> flag for missing cloud pricing comparison (requested in content brief, no upstream source in .sources/)
• Renamed "Execution costs" → "Execution cost formula"; added sentence explaining it maps to the Update message execution and 1B instructions executed rows in the Cost table
• Added cross-reference note in Compute allocation section pointing back to Cost table rows
• Fixed ambiguous storage shorthand: "~329B" → "~329 billion", "~861B" → "~861 billion"
• Rewrote Special features stubs: removed inline <!-- Needs human verification --> comments from list items; added plain text noting costs are "not yet included on this page" and a <!-- TODO --> block for follow-up
• Removed icskills: [] empty array from frontmatter