docs: add current Xquik OpenAPI example

[kriptoburak]

What

• Add English and Chinese OpenAPI examples for Xquik with an explicit --schema-url.
• Document Xquik’s closed-source hosted-service boundary and independence from X Corp.
• Refresh both pinned installer examples from v0.15.3 to the current v0.16.0 release.

Why

Xquik exposes its OpenAPI document separately from the API host, making it a useful real-world --schema-url example. The existing pinned installer commands were 2 releases behind the current package.

How

Both landing-page examples run:

uxc xquik.com --schema-url https://xquik.com/openapi.json -h

The command discovers all 126 current OpenAPI operations without credentials.

Testing

• cargo fmt -- --check
• cargo clippy -- -D warnings
• cargo test --test cli_help_regression_test schema_url_override_supports_schema_separated_openapi_service
• Exact Xquik command returns ok: true and 126 operations
• v0.16.0 release URL returns HTTP 200
• git diff --check

Checklist

• Code formatted
• No clippy warnings
• Relevant tests pass
• English and Chinese documentation updated
• Commit message follows Conventional Commits

Closes

No linked issue.