[Accessibility] document aria-role is needed for Quarto Lens

[jooyoungseo]

After executing Explorer: Focus on Quarto View, user focus moves to article region within VSCode web view where screen reader's virtual cursor (AKA browse mode) is required. Although the parent webview container has "document" role, this attribute is overridden by <article> tag which inherits article role. To fix this, we need to assign "document" role to <article> container.