[DOCS] Fix welcome-to-elastic link (#718)

**Problem:** In elastic/docs#2752, we updated the URL prefix (`welcome-to-elastic`) and name for the "Welcome to Elastic Docs" docs. However, we still have some stray links that use the old `/welcome-to-elastic` URL prefix

**Solution:** Updates an outdated link.

Author: jrodewig

.doc/index-custom-title-page.html

@@ -147,7 +147,7 @@ <h4 class="mt-3">
       </a>
     </div>
     <div class="col-md-4 col-12 mb-2">
-      <a class="no-text-decoration" href="https://www.elastic.co/guide/en/welcome-to-elastic/current/getting-started-observability.html">
+      <a class="no-text-decoration" href="https://www.elastic.co/guide/en/starting-with-the-elasticsearch-platform-and-its-solutions/current/getting-started-observability.html">
         <div class="card h-100">
           <h4 class="mt-3">
             <span class="inline-block float-left icon mr-2" style="background-image: url('https://images.contentstack.io/v3/assets/bltefdd0b53724fa2ce/bltaa08b370a00bbecc/634d9da14e565f1cdce27f7c/observability-logo-color-32px.png');"></span>