[MaxGhenis]

Fixes #2778

axiom: n/a: documentation only

This corrects docs/book/assumptions/growthfactors.md and changes no model code.

• Moves the capital gains variables to a new section on their own index, and the two property values to a new house prices section.

• Adds the 29 names the page omitted, so each of the 127 names in uprating_indices.yaml now sits under its own index.

• Corrects the road fuel table for 2028 to 2031.

• Rewrites the Rent section around what uprate_rent does: private rents by region, every other rent by obr.social_rent. The old table showed obr.rent, which projects no rent.

• Adds sections for Northern Ireland domestic rates, water and sewerage charges, and rail usage, with tables from yoy_growth.yaml.

• States that the OBR stops forecasting mortgage interest after 2029, how council_tax_benefit_reported grows, how employee_pension_contributions_reported grows, and that the student loan routes apply only to new loans.

I checked the result with a script that reads each name in the map and looks for it in the section of its index (127 found, 0 missing), and compared every other table on the page with yoy_growth.yaml to one decimal place.

#2131 and #2638 edit the per capita GDP list too. The second to merge keeps the names from both.

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