Renders the content according to the ``content_format``.

def _render_content(self):
        """Renders the content according to the ``content_format``."""
        if self.content_format == "rst" and docutils_publish is not None:
            doc_parts = docutils_publish(
                source=self.raw_content,
                writer_name="html4css1"
            )
            self.rendered_content = doc_parts['fragment']
        elif self.content_format == "rs" and docutils_publish is None:
            raise RuntimeError("Install docutils to pubilsh reStructuredText")
        elif self.content_format == "md" and markdown is not None:
            self.rendered_content = markdown(self.raw_content)
        elif self.content_format == "md" and markdown is None:
            raise RuntimeError("Install Markdown to pubilsh markdown")
        else:  # Assume we've got html
            self.rendered_content = self.raw_content