Document dataSuffix for Base Pay attribution

[spencerstock]

What changed? Why?

Document the dataSuffix parameter for Base Pay attribution via Builder Codes:

• accept-payments.mdx: Added "Attribution (dataSuffix)" section with usage example and tip about validation
• pay.mdx: Documented dataSuffix param in API reference and added example for payment with attribution
• createBaseAccount.mdx: Updated attribution config docs—replaced "16 byte hex string" with "deterministic hex suffix" and "Custom 0x-prefixed hex string" to reflect no fixed length requirement

How was this tested?

Docs-only change. MDX linter run on changed files (repo has no unit tests for content).

How can reviewers manually test these changes?

1. Run mintlify dev and verify the Accept Payments guide, pay reference, and createBaseAccount reference render correctly
2. Confirm code examples and links to Builder Codes work

Demo/screenshots

[cb-heimdall]

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