chore: add knowledge base link (thirdweb-dev#5147)

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## PR-Codex overview
This PR updates the `supportLinks` array in the `Header.tsx` file to improve the naming of existing links and to add a new link for the Knowledge Base.

### Detailed summary
- Changed the name of the first link from `"Support Site"` to `"Get thirdweb support"`.
- Added a new link for the Knowledge Base with the name `"Knowledge Base"` and the URL `https://support.thirdweb.com`.

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apps/portal/src/app/Header.tsx

@@ -98,9 +98,13 @@ export const connectLinks = [
 
 const supportLinks = [
   {
-    name: "Support Site",
+    name: "Get thirdweb support",
     href: "https://thirdweb.com/support",
   },
+  {
+    name: "Knowledge Base",
+    href: "https://support.thirdweb.com",
+  },
   {
     name: "Contact Sales",
     href: "https://thirdweb.com/contact-us",