Parse splits frontmatter from content and unmarshals the YAML block into out.
When no frontmatter is present, found is false and out is left unchanged.
func Split(content []byte) (yamlBlock []byte, body []byte, found bool)
Split separates a leading YAML frontmatter block (opened and closed by a line
containing exactly "---") from the body. When content has no such block, found
is false and body is the original content unchanged.