docs(UPIIS-57): register UPI Issuance in the API playground

[Anindya-Pandey]

Add the two feeder inputs the playground consumes from this repo, so /payments/upi-issuance resolves there instead of 404ing behind the "Test in API playground" button the docs already render.

• products.json: add UPI Issuance under Payments.
• json/payments/upi-issuance/: 14 mock payloads, one per operation, named after the operationId per this folder's README. Bodies are the spec's existing examples. Each carries sim.plaintext in idempotencyKey, which exchanges the call outside the crypto envelope; an envelope body cannot be authored statically. requestBindingToken and verifyOTP add a scenario directive (sim.bind-ok, sim.otp-ok) for the outcomes that depend on an async event the playground cannot fire. Directives are honoured on sandbox deployments only. No credentials are committed: the sandbox plaintext path needs none.
• upi-issuance.json: operationIds drop the "onboarding#" prefix. '#' is the URL fragment delimiter, so a payload file named after the old form truncates at the '#' and 404s when fetched. Also align three vpa request-body descriptions with the prose pages, which already say the prefix must start with the user's alias rather than their mobile number.

set-alias stays undocumented, so it has no entry here.

Closes UPIIS-57

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