Add the following to LocalSettings.php:

$wgHooks['SkinAfterContent'][]='backlinks_func';

function backlinks_func( &$data, Skin $skin ) {
    $title = $skin->getTitle()->getPrefixedText();
    $wikitext = "{{Special:WhatLinksHere/{$title}}}";

    $parserOptions = ParserOptions::newFromUser( $skin->getUser() );
    $parsedText = MediaWiki\MediaWikiServices::getInstance()->getParser()->parse( $wikitext, $skin->getTitle(), $parserOptions )->getText();

    $out = '<div id="backlinks" class="backlinks"><h2>Backlinks</h2>'. $parsedText .'</div>';

    $data = $out . $data;
    return true;
}

Add a div class to [[:Mediawiki:Common.css]]:

.backlinks {
    background-color: #f9f9f9;
    border: 1px solid #ccc;
    border-radius: 5px;
    padding: 20px;
    margin: 10px 0;
    width: 90%;
    box-sizing: border-box;
}

.backlinks h2 {
    color: #333;
    font-size: 24px;
    font-weight: bold;
    margin-bottom: 10px;
}

.backlinks ul {
    list-style: none;
    padding: 0;
    margin: 0;
}

.backlinks ul li {
    padding: 5px 0;
}

.backlinks ul li a {
    text-decoration: none;
    color: #007bff;
}

.backlinks ul li a:hover {
    color: #0056b3;
    text-decoration: underline;
}

The following could also work by directly querying the database, however, it has some issures surrounding subpages:

$wgHooks['SkinAfterContent'][]='backlinks_func';

function backlinks_func( &$data, Skin $skin ) {
    $dbr = wfGetDB( DB_REPLICA );
    $res = $dbr->select(
      'pagelinks',
      'pl_title',
      array('pl_namespace' => 0, 'pl_from' => $skin->getTitle()->getArticleID())
    );
    $out = '<div id="backlinks" class="backlinks"><h2>Backlinks</h2><ul>';
    foreach( $res as $row ) {
        $title = Title::newFromText($row->pl_title);
        if ($title) {
            $url = $title->getFullURL();
            $out .= "<li><a href='" . htmlspecialchars($url) . "'>" . htmlspecialchars($title->getFullText()) . "</a></li>";
        }
    }
    $out .= '</ul></div>';
    $data = $out . $data;
    return true;
}