Create a document in one of two explicit modes.

Content mode (the default) writes the document you pass, byte for byte: frontmatter
first, then the markdown, normally starting with the title heading. The key is
required -- it is the document's identity.

Template mode (--template NAME) composes the document from a named template in the
configuration. The template name is always required. Variables come from --var,
--vars-yaml and --vars-json, frontmatter fields from --set, and the key is derived
from the template's key_template unless you pass one.

--content and --template are mutually exclusive.
