docs: (backport) Bring the main doc pages up to date, and add more examples (#2094)

docs/index.yml

@@ -18,7 +18,13 @@
       path: ping.js
     - name: Avatars
       path: avatars.js
+    - name: Attachments
+      path: attachments.md
     - name: Server greeting
       path: greeting.js
+    - name: Message Embed
+      path: embed.js
+    - name: Moderation
+      path: moderation.md
     - name: Webhook
       path: webhook.js