Add social media meta info

[greggman]

It would be nice if, when sharing a link, a thumbnail appeared.

The thumbnail should be 1200x630

Here's some example code to put into the the root index.html

<meta name="description" content="webgpu samples">
<meta name="keywords" content="webgpu graphics">
<meta name="thumbnail" content="https://webgpu.github.io/webgpu-samples/path/to/thumbnail.jpg">

<meta property="og:title" content="WebGPU Samples">
<meta property="og:type" content="website">
<meta property="og:image" content="https://webgpu.github.io/webgpu-samples/path/to/thumbnail.jpg">
<meta property="og:description" content="WebGPU Samples">
<meta property="og:url" content="https://webgpu.github.io/webgpu-samples/">

<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:domain" content="webgpu.github.io">
<meta name="twitter:title" content="WebGPU Samples">
<meta name="twitter:url" content="https://webgpu.github.io/webgpu-samples/">
<meta name="twitter:description" content="WebGPU Samples">
<meta name="twitter:image:src" content="https://webgpu.github.io/webgpu-samples/path/to/thumbnail.jpg">