[Feature Request] Add Algolia DocSearch version meta tag

There is already the language meta tag in `layout.html`:

```html
<meta name="docsearch:language" content="{{ language }}"/>
```

Is it possible to also add the version meta tag e.g.

```html
<meta name="docsearch:version" content="{{ version }}"/>
```

With this it will be possible to filter results for specific versions using Algolia DocSearch.