docs: add event and payment testing flow

[RathodDarshil]

Summary

• Add a standalone guide for choosing between trackEvent and capturePayment
• Add an end-to-end test flow for custom events, payment deduplication, Meta forwarding, and payment removal
• Add the page to SDK Guides and cross-link it from both event API pages

Verification

• npm run check:routes passes with 90 valid pages
• The new page compiles with @mdx-js/mdx
• npx mintlify broken-links reports 29 pre-existing broken links and none from this change
• The installed Mintlify CLI does not expose the repository's configured mintlify build command, so a full local Mintlify build could not be run