docs(users): add Slab to the users list (#3800)

Author: luin

website/siteConfig.js

@@ -120,6 +120,11 @@ const users = [
         image: '/img/rocketchat_logo.svg',
         infoLink: 'https://rocket.chat/',
     },
+    {
+        caption: 'Slab',
+        image: '/img/slab-logo.png',
+        infoLink: 'https://slab.com/',
+    },
     {
         caption: 'Slides',
         image: 'https://s3.amazonaws.com/uploads.uservoice.com/logo/design_setting/116173/original/slides-symbol-150x150.png',