Serializes a markermd_template to a human-readable, schema-validatable
YAML file. The assignment document is referenced by source_path (recorded
under source.path) rather than embedded; the parsed AST is regenerated on
load by re-parsing that document.
Examples
if (FALSE) { # \dontrun{
tmpl = read_template_yaml("hw01-template.yaml")
write_template_yaml(tmpl, "hw01-template-copy.yaml")
} # }