Add product intro video to README

[terrywangcode]

Summary

• add the public OpenHive product intro video asset
• embed the playable video near the top of the English README
• sync the same video placement and copy in README.zh-CN.md

Testing

• not run; documentation and media asset only

[gemini-code-assist]

Code Review

This pull request adds a product introduction video to the project's README files in both English and Chinese. The reviewer recommended enhancing the video implementation for better accessibility and browser compatibility by using the tag and adding aria-label attributes.

[gemini-code-assist]

To improve accessibility and browser compatibility, consider using a <source> tag and adding an aria-label. Additionally, adding a poster attribute with a thumbnail image would improve the visual experience while the video is loading.

Suggested change

	<video src="docs/assets/openhive-product-intro.mp4" controls playsinline width="100%" title="OpenHive product introduction"></video>
	<video controls playsinline width="100%" title="OpenHive product introduction" aria-label="OpenHive product introduction">
	<source src="docs/assets/openhive-product-intro.mp4" type="video/mp4">
	Your browser does not support the video tag.
	</video>

[gemini-code-assist]

为了提高可访问性和浏览器兼容性，建议使用 <source> 标签并添加 aria-label。此外，添加 poster 属性（缩略图）可以改善视频加载前的视觉体验。

Suggested change

	<video src="docs/assets/openhive-product-intro.mp4" controls playsinline width="100%" title="OpenHive 产品介绍"></video>
	<video controls playsinline width="100%" title="OpenHive 产品介绍" aria-label="OpenHive 产品介绍">
	<source src="docs/assets/openhive-product-intro.mp4" type="video/mp4">
	您的浏览器不支持 video 标签。
	</video>