Add Benefits-in-Kind planning docs page (#1654)

[vahid-ahmadi]

Summary

• Closes Plan: model benefits-in-kind (BiK) — scope, implementation, and data needs #1654 (the planning tracker).
• Plan: model benefits-in-kind (BiK) — scope, implementation, and data needs #1654 was filed as a planning issue but lived only in GitHub. Persist the agreed plan in-repo as a "planned: not yet modelled" page at docs/book/programs/gov/hmrc/benefits-in-kind.md so future work has a stable reference rather than a moving issue body.
• The page covers:
  • Current state — the integration hook in variables/household/benefits/employment_benefits.py is explicitly documented as the slot future BiK variables plug into.
  • Phase 1 MVP scope — company car, car fuel, private medical, living accommodation, beneficial loans.
  • Phases 2 & 3 and the exempt-from-charge out-of-scope set.
  • Implementation plan — proposed variables under variables/gov/hmrc/income_tax/bik/, parameter tree layout, and knock-on effects through ANI / PA taper / HICBC / Class 1A NICs.
  • Data needs — the FRS-flag-but-no-value gap per category, plus calibration sources (HMRC BiK statistics, Company Car Tax statistics, SPI tables, DVLA/SMMT).
  • Imputation approach mirroring the second-stage-imputation pattern from Align UK model/data pipeline with US patterns: reported-anchoring, second-stage imputation, and downstream bugs #1621.
  • Effort estimate and open questions (Class 1A NICs, UC treatment, Datalab access).
• When the MVP lands, the "planned" warning at the top is dropped and the file becomes the live programme docs page.

Test plan

• Markdown file lints cleanly (table rendering, code blocks, MyST directives).
• Docs build job picks up the new page under programs/gov/hmrc/.

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