Add PIP point system planning doc (#1069)

[vahid-ahmadi]

Summary

• PIP point system #1069 references SI 2013/377 Part 2 (daily-living activities and points) and asks for the PIP point system to be modelled.
• Currently PolicyEngine UK reads the category (STANDARD / ENHANCED / NONE) directly via pip_dl_category and pip_m_category and multiplies by the published weekly rate. That's correct for population aggregates because the FRS doesn't capture activity-level scores and DWP's caseload publications report bands rather than scores.
• The points machinery would matter for three reform scenarios the categorical model can't answer: descriptor reforms (e.g. the 2024 Green Paper proposals), threshold reforms (raising the 8/12 cutoffs), and any analysis that distributes by activity.
• Three-phase plan in the new doc at docs/book/programs/gov/dwp/pip-points-plan.md:
  1. Points-to-category mapping behind a use_point_system switch parameter (10 daily-living + 2 mobility activity scores -> total -> band via threshold params), preserving the categorical default.
  2. Descriptor-level parameters under gov/dwp/pip/descriptors/ so descriptor point values themselves are reformable.
  3. Data-side descriptor imputation in policyengine-uk-data (needed to make Phase 2 reforms work on the calibrated microsim).
• Pure planning; current PIP variables and parameters unchanged.

Test plan

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

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