Update resource links in images of text (#4311)

1. remove 404ing links
2. remove links to out of date content
3. add in MDN links on text and font styling and web fonts.

Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: fstrr

understanding/20/images-of-text.html

@@ -132,10 +132,8 @@ <h2>Examples of Images of Text</h2>
    <section id="resources">
       <h2>Resources for Images of Text</h2>
       <ul>
-         <li><a href="http://alistapart.com/article/cssatten">CSS Web fonts</a></li>
-         <li><a href="http://clagnut.com/blog/2042/">Weblog comments: WebKit now supports CSS @font-face rules</a></li>
-         <li><a href="http://www.workingwith.me.uk/articles/css/cross-browser-drop-shadows">Creating Cross Browser Compatible CSS Text Shadows</a></li>
-         <li><a href="http://www.yourhtmlsource.com/stylesheets/csstext.html">CSS and text</a></li>
+        <li><a href="https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Text_styling/Fundamentals">Fundamental text and font styling</a></li>
+        <li><a href="https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts">MDN Web fonts</a></li>
       </ul>
    </section>