Update demo label styling in navigation bar

Replaced the italicized text with a dark badge for the "demo" label in the navigation bar. This improves clarity and visual consistency within the UI design.

Author: khalidabuhakmeh

src/Pages/Shared/_Nav.cshtml

@@ -13,7 +13,7 @@
         <a href="~/" class="navbar-brand py-2 d-md-inline-block">
             <img src="~/duende-white.svg" class="icon-banner" alt="Duende IdentityServer"/>
             <span class="text-white">IdentityServer</span>
-            <small class="text-white-50 font-italic">(demo.duendesoftware.com)</small>
+            <span class="badge badge-dark">demo</span>
         </a>
         <a class="text-white py-3 d-none d-md-inline-block" href="/#features">Features</a>
         <a class="text-white py-3 d-none d-md-inline-block" href="/#references">References</a>