Issue #3 Expand the sample wiki with one more real-world gotcha

[amandeepsingh29]

Fixes #3

Hey! I added a new gotcha page for Payment Idempotency Keys to the sample repo so it feels a bit more like a real-world wiki.

To make the example actually tie together, I also tweaked the dummy authorizePayment function in src/payments.ts to accept an idempotencyKey parameter. That way, the new docs actually match the sample code they're pointing to.

What's included:

• Created payment-idempotency-keys.md with the required frontmatter and a link to [[src/payments.ts]].
• Updated src/payments.ts to take the new parameter.
• Tested it locally to make sure almanac search picks it up properly from inside examples/sample-repo.

Here's the search working locally:

Let me know if you need any changes!