Add child maintenance planning docs page (#669)

[vahid-ahmadi]

Summary

• Child maintenance #669 is filed as a planning tracker (CSJ paper on the parent-poverty trap, plus gov.uk's CMS methodology PDF). Persist the agreed scope in-repo as a "planned: not yet modelled" page mirroring the BiK / firm-VAT pattern.
• New page at docs/book/programs/gov/cms/child-maintenance.md covering:
  • the current state — generic maintenance_income / maintenance_expenses inputs flowing through market_income and HBAI, but not distinguishing child from spousal maintenance and not encoding the UC asymmetry,
  • the UC asymmetry flagged by the CSJ Hidden Parent Poverty Trap paper — recipient gets a full disregard, payer gets no deduction — and the work-incentive cliff it produces,
  • Phase 1: disambiguate inputs into child_maintenance_received / child_maintenance_paid (plus the FRS routing needed in policyengine-uk-data),
  • Phase 2: fix the UC asymmetry in uc_applicable_income,
  • Phase 3 (optional): derive the CMS formula itself for reform analysis, with the gross-income-band rate table reproduced from SI 2012/2677,
  • data needs (FRS for inputs, CMS statistics for calibration) and primary references — Child Support Act 1991, SI 2012/2677, the DWP "how we work out child maintenance" PDF, and the CSJ paper.
• Pure documentation; no parameter or variable changes.

Test plan

• Markdown lints cleanly (table, admonition, link references).
• Docs build job picks up the new page under programs/gov/cms/.

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