Show page's description (page.rowDescription) in the TYPO3 backend page module (based on EXT:sys_note)

ext_localconf.php

<?php

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['cms/layout/db_layout.php']['drawHeaderHook']['my_theme'] = 
    \MyVendor\MyTheme\Hook\PageRowDescriptionHook::class . '->render';
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['cms/layout/db_layout.php']['drawFooterHook']['my_theme'] = 
    \MyVendor\MyTheme\Hook\PageRowDescriptionHook::class . '->render';

PageRowDescriptionHook.php

<?php
namespace MyVendor\MyTheme\Hook;

use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Fluid\View\StandaloneView;
use TYPO3\CMS\Fluid\ViewHelpers\Be\InfoboxViewHelper;

class PageRowDescriptionHook
{

    /**
     * Add page's rowDescription as additional content to the header and footer of the page module
     *
     * @param array $params
     * @param \TYPO3\CMS\Backend\Controller\PageLayoutController $parentObject
     * @return string
     */
    public function render(array $params = [], \TYPO3\CMS\Backend\Controller\PageLayoutController $parentObject)
    {
        $page = $parentObject->pageinfo;
        if ($page && $page['rowDescription']) {
            $view = GeneralUtility::makeInstance(StandaloneView::class);
            $view->setTemplatePathAndFilename(GeneralUtility::getFileAbsFileName('EXT:backend/Resources/Private/Templates/InfoBox.html'));

            $title = 'Page description';
            $message = '<p>' . nl2br(htmlspecialchars($page['rowDescription'])). '</p>';
            $view->assignMultiple([
                'title' => $title,
                'message' => $message,
                'state' => InfoboxViewHelper::STATE_INFO
            ]);

            return $view->render();
        }

        return '';
    }
}