Add a maintenance banner

Author: alexcrichton

_includes/header.html

@@ -30,3 +30,6 @@
           {%- endif -%}
         </div>
       </header>
+<div class="wrapper" style="background:rgb(245, 170, 170);border: 5px solid red;">
+  This project and website is <a href="https://blog.rust-lang.org/inside-rust/2025/07/21/sunsetting-the-rustwasm-github-org/">no longer maintained</a>
+</div>