chore: Update welcome.mdx to use Slack instead of Discord (#70)

Author: Ryan-Amirthan

fern/docs/pages/welcome.mdx

@@ -12,11 +12,15 @@ Welcome to the Plant Store API docs! This site demonstrates how to use Fern to g
 Here you'll find information about managing your plants, customers, and orders for your plant shop.
 
 <CardGroup>
-  <Card title="API Reference" icon="fa-solid fa-code" href="/api-reference" />
+  <Card 
+    title="API Reference" 
+    icon="fa-solid fa-code" 
+    href="/api-reference" 
+  />
   <Card
-    title="Discord Community"
-    icon="fa-brands fa-discord"
-    href="https://discord.com/invite/JkkXumPzcG"
+    title="Slack Community"
+    icon="fa-brands fa-slack"
+    href="https://join.slack.com/t/fern-community/shared_invite/zt-2q7ev4mki-mhO5anKslwRowp4oExWf4A"
   />
   <Card
     title="Blog"