Add a note to use the desktop builds when seshat isn't available

Fixes https://github.com/vector-im/element-web/issues/15184

This is currently temporary design for https://github.com/vector-im/element-web/issues/12896 but does not fix it.

src/SdkConfig.ts

@@ -33,6 +33,11 @@ export const DEFAULTS: ConfigOptions = {
         // Default conference domain
         preferredDomain: "jitsi.riot.im",
     },
+    desktopBuilds: {
+        available: true,
+        logo: require("../res/img/element-desktop-logo.svg"),
+        url: "https://element.io/get-started",
+    },
 };
 
 export default class SdkConfig {