Update Business Onboarding API docs to match implementation

[alexBinns]

Summary

• The Business Onboarding API docs had drifted from kari's createOnboardingRequest.route.js: documents new request fields (sendInvitation, merchant.redirectUrls, merchant.searchHidden), corrects settlementAccountNumber/externalMerchantId from required to optional, notes deviceId is only required when vendor isn't nomad, and adds the LIVENESS_MISMATCH error code.
• Adds a second worked example (e-commerce merchant with a Shopify sales channel) alongside the existing physical merchant example, since e-commerce merchants use redirectUrls instead of location.

Manual test steps

1. Run the docs site locally and navigate to the Business Onboarding API page.
2. Confirm the "Create an Onboarding Request" section renders the request/response panel for the physical merchant example, followed immediately by an "E-commerce Example" subsection with its own request/response panel — both under one nav entry in "On this page".
3. Confirm the Onboarding Request Model, Merchant Model, Asset Program Model, and Sales Channel Model property tables render without markup errors.
4. Confirm the new LIVENESS_MISMATCH error response renders correctly in the Error Responses list.

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