gettemplate.php

// Custom Templates Shortcode to Get Templates from the /templates Directory
// Usage: [gettemplate name="templatename"]

function prefix_templates_shortcode_function( $atts ){
  $a = shortcode_atts( array(
    'name' => 'none'
  ), $atts );
  ob_start();
  get_template_part('templates/' . $a['name']);
  return ob_get_clean();
}
add_shortcode('gettemplate', 'prefix_templates_shortcode_function');