docs(payments): add crypto settlement step to Merchant Quickstart

[riveign]

Summary

• Adds a new Step 4: Configure crypto settlement to the Merchant Quickstart, between Create your first merchant and Create your first payment.
• The step shows a POST /v1/merchants/{merchantId}/settlements/crypto example registering USDC on Base, explains the CAIP-19 asset / CAIP-10 destination pairing, and links to Token & Chain Coverage for the full list of valid asset IDs.
• Calls out the (merchant, asset) uniqueness rule + points to the Update endpoint for changing a destination.
• Renumbers Create your first payment → Step 5 and Check the payment status → Step 6; updates the intro from "five steps" to "six steps".

Why

Settlement configuration was missing from the original Quickstart (shipped as PR #50). Without it, a freshly created merchant has no settlement destination registered, so POST /v1/payments doesn't have a place to deliver funds. This restores the missing prerequisite.

Test plan

• mint dev preview renders the new Step 4 in the correct slot
• Inline links resolve: Token & Chain Coverage, Create crypto settlements, Update a crypto settlement
• Numbered list at top of page reads 1–6 with correct titles
• curl example copies cleanly with \ line continuations intact

🤖 Generated with Claude Code

[mintlify]

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Project	Status	Preview	Updated (UTC)
wc-pay-docs	🟢 Ready	View Preview	May 19, 2026, 3:03 PM

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