Document RF nowcasting methodology (#1400 #1179)

[vahid-ahmadi]

Summary

• Follow-up to Add page to documentation on differences between our nowcasting and RF's nowcasting #1171's high-level comparison page. This is the deeper appendix focused on what RF does rather than how it compares to PolicyEngine, drawing from the Living Standards Outlook 2025 appendix and the Black Holes and Consolidations methodology notes.
• New standalone page at docs/book/assumptions/rf-nowcasting-methodology.md covering:
  • Earnings growth (Add RF methodology for earnings growth #1179): explicit wage floor, spillover effect, 2024-25 NLW extension to 21-22-year-olds, provisional 2029-30 extension to 18-20-year-olds, equal employee/self-employed uprating beyond the floor.
  • Housing costs: PIPR for private rents, social-rent policy for social, OBR base-rate path for mortgage interest.
  • Benefits: UC-statistics caseload anchoring and historical take-up rates.
  • Inflation indices: EFO CPI/CPIH/RPI held flat beyond the forecast horizon.
  • Horizon: 4-5 fiscal years ahead.
• Cross-links to growthfactors.md (PolicyEngine's own growth rates) and nowcasting-comparison.md (the side-by-side comparison page from Add page to documentation on differences between our nowcasting and RF's nowcasting #1171), so the three docs function as a coherent set.

Closes #1400 and #1179.

Test plan

• Markdown lints cleanly (no broken cross-link anchors).
• Docs build job picks up the new page under assumptions/.

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