layout/404: link to where to file issues on dead urls

This makes sure people know where to report problems with dead URLs,
which seems like a good thing.

layouts/404.html

 {{ define "main"}}
-<main class="container my-3 text-center">
-  <h1 class="display-2">404</h1>
-  <p class="lead">Page not found :(</p>
-  <p>The requested page could not be found.</p>
+<main class="container my-3">
+  <div class="text-center">
+    <h1 class="display-2">404</h1>
+    <p class="lead">Page not found :(</p>
+    <p>The requested page could not be found.</p>
+  </div>
+
+  <div class="alert alert-warning">
+    If you believe this is a mistake on our behalf, please
+    <a class="alert-link" href="https://gitlab.freedesktop.org/mesa/mesa3d.org/-/issues/new">
+    file a ticket</a>, and we'll take a look. Make sure to include the
+    full URL of the page you're currently viewing, as well as the URL of
+    the page that led you here, or any other explanation as to why you
+    believe this is a mistake.
+  </div>
 </main>
 {{ end }}