Renders the metadata value with the given key from the current document.
The metadata value will be rendered as a string. If no value exists with the specified key, nothing will be rendered. In addition to using the shortcode by the
<?# Meta key /?>, this shortcode can also be used with a special syntax:
<?#= key /?>.
The following parameters can be used without names in the order below or with names (provided they are named) in any order. When using named parameters, the syntax should be `Name=Value` or `Name="Value"`.
The key of the metadata value to render.