Expand National Insurance documentation page (#746)

[vahid-ahmadi]

Summary

• Closes Documentation page for National Insurance #746.
• The existing page covered Classes 1, 2, and 4, mentioned Class 3 only in passing, and ended in two appendix charts. The 2022 issue asked for a page "similar to the current pages" — this rounds it out so it sits alongside the income-tax and child-benefit pages.
• Additions:
  • Class 3 (voluntary) row in the modelling table — it's now modelled via ni_class_3.
  • New "How PolicyEngine computes National Insurance liability" section that walks through the per-class variables (ni_class_1_employee, ni_class_1_employer, ni_class_2, ni_class_3, ni_class_4) and explains how they roll up into national_insurance and government revenue.
  • References section linking SSCBA 1992, the HMRC rates page, and HMRC outturn statistics.
• Drive-by fix: the rates chart cell was doing df[df[\"Threshold\"] == False][df.Value < 1], which produced a pandas "Boolean Series key will be reindexed" UserWarning in the rendered output. Combined into a single boolean mask.

Test plan

• Notebook still valid JSON.
• Docs build job renders the new sections cleanly and without the pandas warning.

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