Support static HTML from MDX content

An `mdx` field delivers an object with `raw` input and `code` output, which can be passed to a Contentlayer method to render it as a component.

It would be great to also have the HTML code (without interactivity or the ability to hydrate).

I would personally like this as a way to use Contentlayer outside a dynamic/server-side context (i.e. in a non-Next project). But we've also heard a scenario in which someone may want to render content statically to an XML feed where the code/interactivity is irrelevant, but the content (along with the semantic markup) is important.