Expand Universal Credit documentation page (#745)

[vahid-ahmadi]

Summary

• Closes Documentation page for Universal Credit #745.
• The 2022 issue asked for a UC docs page like the existing Child Benefit one. The page already exists but stopped at "here are some element rates over time" — round it out so it explains the full computation.
• Additions:
  • New "How PolicyEngine computes Universal Credit" section: uc_maximum_amount -> means test (work allowance + taper + asset rules) -> benefit cap reduction, plus a note on how would_claim_uc handles take-up.
  • References section linking the Welfare Reform Act 2012, the operational regs (SI 2013/376), the annual DWP rate uprating publication, and the UC official statistics page used for calibration.
• Drive-by fixes in the rates chart:
  • Rebuilt the dataframe in one pass instead of pd.concat inside a loop, which was raising a FutureWarning in the rendered output.
  • Amount(£m) y-axis label is wrong — UC elements are £/month, not millions; corrected to Amount (£/month).
  • Removed an empty trailing code cell.

Test plan

• Notebook still valid JSON, 5 cells (down from 4 + 1 empty).
• Docs build job renders the new sections cleanly.

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