Stacked on top of #30121.
This is the same thing we do for `renderToReadableStream` so that you
don't have to manually inject it into the stream.
The only reason we didn't for `renderToString` / `renderToStaticMarkup`
was to preserve legacy behavior but since this is a new API we can
change that.
If you're rendering a partial it doesn't matter. This is likely what
you'd do for RSS feeds. The question is if you can reliably rely on the
doctype being used while rendering e-mails since many clients are so
quirky. However, if you're careful it also doesn't hurt so it seems best
to include it.
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