Update IdentityServer demo card content for clarity

Refined the demo card description to provide more context about the IdentityServer demo's capabilities. Emphasized testing OAuth 2.0, OpenID Connect flows, and token usage to enhance user understanding.

Author: khalidabuhakmeh

src/content/docs/index.mdx

@@ -42,8 +42,10 @@ import Testimonial from "~/components/testimonial.astro";
         ```
     </Card>
     <Card title="Demo Server" icon="setting" href="https://demo.duendesoftware.com">
-        Try out our IdentityServer demo server.
-        <LinkButton href="https://demo.duendesoftware.com" variant="secondary">
+      Experience IdentityServer in action with our live demo server. Test OAuth 2.0 and OpenID Connect flows, explore
+      client configurations, and see how security tokens work in practice.
+
+      <LinkButton href="https://demo.duendesoftware.com" variant="secondary">
             Try it out
         </LinkButton>
     </Card>