docs: add Xquik OpenAPI quick start

[kriptoburak]

Summary:

• Add a hosted authenticated-spec quick start using Xquik's public OpenAPI document.
• Keep authentication opt-in with an environment placeholder.

Validation:

• git diff --check
• curl https://xquik.com/openapi.json source check
• go test ./...

[andig]

Thank you for this PR. I'm not sure if its in scope to add this specific example to the README though?

[kriptoburak]

If this is useful and you merge it after checks pass, feel free to share the PR on X/Twitter and mention my handle, burakbayir. I can repost or retweet it to my 24k followers of developers and power users, which may help more relevant users discover this repo.

[andig]

Thanks, but bot looking for publicity ;)