DEV: Add plugin outlet above the subscriptions' subscribe title (#36501)

This commit adds a plugin outlet, `above-subscriptions-subscribe-title`
above the heading. There are no outlet arguments for this plugin outlet.

This was created from the question in:
https://meta.discourse.org/t/insert-intro-text-on-s-page/390569

---------

Co-authored-by: Jarek Radosz <jarek@cvx.dev>

plugins/discourse-subscriptions/assets/javascripts/discourse/templates/subscribe.gjs

@@ -1,7 +1,10 @@
+import PluginOutlet from "discourse/components/plugin-outlet";
 import { i18n } from "discourse-i18n";
 
 export default <template>
   <div class="container">
+    <PluginOutlet @name="above-subscriptions-subscribe-title" />
+
     <div class="title-wrapper">
       <h1>
         {{i18n "discourse_subscriptions.subscribe.title"}}