docs: correct lease vendor namespace segment count

[narutamaaurum]

Summary

• fix docs/guides/leases.md to say x-vendor. names need at least two segments after the prefix
• link the leases guide back to the vendor-extensions guide as the namespace source of truth
• add the canonical x-vendor.opentelemetry.tracecontext example in nearby prose so the two guides stay aligned

Validation

• git diff --check
• reviewed docs/guides/vendor-extensions.md against packages/core/src/extensions.ts
• full workspace lint/typecheck/test not run in this runner because the repo requires Node >=22 while this environment has Node v12.22.9

Closes #85