[EDU-1941] Add a guide for building livestream chat applications (#2592)

* Add a guide for livestream chat applications

* chat: refactor livestream guide

This change refactors the livestream guide to take into account feedback from the initial edition. The change includes:

- More diagrams
- More technical depth on decision points
- A stronger focus on how Ably's architecture works and the USPs of the product

* chat: add review changes to use-case guide

---------

Co-authored-by: Andy Ford <[email protected]>

Author: m-hulbert

src/data/nav/chat.ts

@@ -118,6 +118,15 @@ export default {
         },
       ],
     },
+    {
+      name: 'Guides',
+      pages: [
+        {
+          name: 'Livestream chat',
+          link: '/docs/guides/chat/build-livestream',
+        },
+      ],
+    },
   ],
   api: [
     {