#672 asks for a comparison of PolicyEngine UK's pension treatment with
the Scottish Tax-Benefit Model's methodology described in the Virtual
Worlds blog post.

Persist the comparison in-repo as a docs page. The headline conclusion
is the two models compute the same aggregate fiscal cost of pension
tax relief for any given household; they differ only in presentation:

- STBM splits relief-at-source (basic rate) and the higher-rate top-up
  (via SA basic-rate-band extension);
- PolicyEngine UK combines them into a single pension_contributions_relief
  reduction in taxable income — matching the HMRC pension tax relief
  statistics methodology used as the calibration target.

The page also enumerates:

- shared mechanics (annual allowance taper, salary sacrifice, employer
  contribution exclusion from earnings),
- shared gaps (annual-allowance carry-forward not modelled in either,
  decumulation taxation beyond basic private_pension_income),
- divergences in State Pension structure (PolicyEngine UK uses the
  three-variable BSP / NSP / ASP split; STBM uses a single transfer
  per the linked blog),
- post-2024 lifetime-allowance abolition: historical LTA params
  retained for back-cast, post-2024 lump-sum allowances not yet
  modelled.

Cross-links to the pension-tax-relief and state-pension docs pages
added under #704 and #1632 respectively, so the three pages function
as a coherent pension-modelling reference.