docs: add Knowledge Base link on getting started page (#11560)

<!--
Please inform yourself about the contribution guidelines on submitting a
PR here:
https://github.com/zitadel/zitadel/blob/main/CONTRIBUTING.md#submit-a-pull-request-pr.
Take note of how PR/commit titles should be written and replace the
template texts in the sections below. Don't remove any of the sections.
It is important that the commit history clearly shows what is changed
and why.
Important: By submitting a contribution you agree to the terms from our
Licensing Policy as described here:
https://github.com/zitadel/zitadel/blob/main/LICENSING.md#community-contributions.
-->

# Which Problems Are Solved
- Added a new "Knowledge Base" card to the Get Started section.

# How the Problems Are Solved
- Updating the index.mdx file with [Knowledge Base
Link](https://help.zitadel.com/)

Co-authored-by: Rajat Singh <[email protected]>

apps/docs/content/index.mdx

@@ -4,7 +4,7 @@ description: ZITADEL is the "Identity Infrastructure for developers". We provide
 ---
 
 import { Card, Cards } from 'fumadocs-ui/components/card';
-import { Rocket, Laptop, BookOpen, Book, Server, ShieldCheck, Terminal, MessageCircle, Calendar, Heart } from 'lucide-react';
+import { Rocket, Laptop, BookOpen, Book, Server, ShieldCheck, Terminal, MessageCircle, Calendar, Heart, HelpCircle } from 'lucide-react';
 
 **ZITADEL is the Identity Infrastructure for Developers.**
 
@@ -38,6 +38,12 @@ Explore our guides to get up and running with ZITADEL quickly.
     icon={<Laptop />}
     description="Sample applications to help you get started."
   />
+  <Card
+    title="Knowledge Base"
+    href="https://help.zitadel.com"
+    icon={<HelpCircle />}
+    description="Find answers to common ZITADEL questions and issues."
+  />
 </Cards>
 
 ## Deployment Options