fix GitHub ribbon (#1291)

The old GitHub ribbon src doesn't seem to be working anymore.
I've adjusted it from the official GitHub blog:
https://github.blog/2008-12-19-github-ribbons/

docs/_includes/masthead.html

@@ -1,6 +1,6 @@
 <div class="masthead">
   <a href="https://github.com/TNG/ArchUnit" class="only-on-fullhd">
-    <img style="position: absolute; top: 0; right: 0; border: 0;" src="https://camo.githubusercontent.com/a6677b08c955af8400f44c6298f40e7d19cc5b2d/68747470733a2f2f73332e616d617a6f6e6177732e636f6d2f6769746875622f726962626f6e732f666f726b6d655f72696768745f677261795f3664366436642e706e67" alt="Fork me on GitHub" data-canonical-src="https://s3.amazonaws.com/github/ribbons/forkme_right_gray_6d6d6d.png">
+    <img style="position: absolute; top: 0; right: 0; border: 0;" decoding="async" width="149" height="149" src="https://github.blog/wp-content/uploads/2008/12/forkme_right_gray_6d6d6d.png?resize=149%2C149" class="attachment-full size-full" alt="Fork me on GitHub" loading="lazy" data-recalc-dims="1">
   </a>
   <div class="masthead__inner-wrap">
     <div class="masthead__menu">

docs/_pages/about.md

@@ -9,5 +9,4 @@ as plain unit tests.
 
 It was originally created by [Peter Gafert](https://github.com/codecholeric) to tackle shortcomings 
 of existing free tools / libraries that assert architectural constraints of Java projects. 
-ArchUnit is mainly developed by [TNG](https://github.com/TNG), but any contributor is welcome 
-and contributions are greatly appreciated :-)
+Any contributor is welcome and contributions are greatly appreciated :-)