fix: update iframe styling for improved responsiveness in index.md

docs/zh/demo/index.md

-<iframe src="https://opendatalab-mineru.ms.show" frameborder="0" width="850" height="850"></iframe>
+<iframe src="https://opendatalab-mineru.ms.show" style="min-height: calc(-40px + 100vh); width: 100%; flex-grow: 1; border: medium; overflow: auto; height: 1746px;"></iframe>
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