docs: add Xquik OpenAPI source example

[kriptoburak]

Summary

• Add a Xquik OpenAPI source example beside the existing Petstore example.
• Clarify the global install command examples so each package manager command is complete.

Checks

• git diff --check
• curl -fsSL --range 0-300 https://xquik.com/openapi.json | head -c 300
• Not run: bun run format:check because this shallow checkout does not have oxfmt installed without bootstrap.