Adding footer credit for background

Author: AndyA13

src/components/Footer.astro

@@ -7,6 +7,7 @@
     <a href="/tags/">🏷️ Tags</a>
     <a href="/rss.xml">📡 RSS</a>
   </nav>
+  <p>Background by <a href="https://unsplash.com/@loganvoss" target="_blank" rel="noopener noreferrer">Logan Voss on Unsplash</a></p>
   <p>&copy; {new Date().getFullYear()} Andrew Aitken</p>
 </footer>