$uri = parse_url($_SERVER['REQUEST_URI'] ?? '', PHP_URL_PATH);
if ($uri === '/about' || $uri === '/about/') {
    header('Content-Type: text/html; charset=UTF-8');
    readfile(__DIR__ . '/about/index.html');
    exit;
}
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://www.nursingmgt.com/wp-sitemap-index.xsl" ?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"><sitemap><loc>https://www.nursingmgt.com/wp-sitemap-posts-post-1.xml</loc></sitemap><sitemap><loc>https://www.nursingmgt.com/wp-sitemap-posts-page-1.xml</loc></sitemap><sitemap><loc>https://www.nursingmgt.com/wp-sitemap-taxonomies-category-1.xml</loc></sitemap><sitemap><loc>https://www.nursingmgt.com/wp-sitemap-users-1.xml</loc></sitemap></sitemapindex>