docs(v2): describe Money wrapper shape on partner-api responses

[samuelwisemanhx]

What does this PR do? (please provide any background)

Updates the Currency and Pricing section of site/partner/v2/api-endpoint-navigation-guide.md to describe the new { amount_major, currency } Money wrapper shape that partner-api v2 emits for every money-bearing field. Removes the previous claim that values are in minor currency units; adds a worked example and explains the optional-money null contract.

Pairs with holidayextras/partner-api#471 (the actual response-shape change).

Any tech debt?

None added.

Screenshots / Screencast

n/a — markdown change. Diff visible in the file change tab.

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