Improve documentation and contents of the 404 page

<?php

/*

    Theme: Fachschaft Computerlinguistik

    404 Template File

/**

 * Template for the error 404 (Not Found) page.

 * 

 * WordPress identifies this file by its name (which MUST be 404.php)

 * and displays it whenver an HTTP 404 is sent.

 * 

 * @see https://codex.wordpress.org/Creating_an_Error_404_Page

 */

get_header();

?>

<main>

    <article>

        <div class="post-header">

        </div>

        <div class="post-header"></div>

        <div class="post-title">

            <h1 class="post-title-box">Error: 404</h1>

            <h1 class="post-title-box">Fehler: 404</h1>

        </div>

        <div class="post-content error404">

            <div class="icon">

                ?!

            </div>

            <div class="message">

                Die angefragte Seite existiert leider nicht.

            </div>

            <div class="icon">?!</div>

            <div class="message">Die angefragte Seite existiert leider nicht.</div>

        </div>

        <div class="post-meta">

        </div>