Add custom content to cart and checkout pages with an action hook

cart checkout hook

//ADD BANNER TO CART PAGE
add_action('woocommerce_before_cart', 'madcow_above_cart_and_checkout_message');
add_action('woocommerce_after_order_notes', 'madcow_above_cart_and_checkout_message');
function madcow_above_cart_and_checkout_message() {
    echo 'your-content-here';
}