Add dynamic typing animation to homepage description

Introduced a dynamic typing effect for the homepage description using the `Typed` library. Replaced hardcoded description text in `Index.razor` with a dynamically rendered `<span>` element. Added a constant for the description text and implemented `OnAfterRenderAsync` to invoke the typing animation on first render.

Included the `typed.umd.js` library in `index.html` and updated `receiver-signature.js` with a `startTyped` function to support typing animations. Registered `startTyped` in the public API for external use.

Enhanced the user experience with dynamic content rendering and improved maintainability by centralizing the description text. Also added a new theme file for improved styling.

EnvelopeGenerator.ReceiverUI/wwwroot/index.html

@@ -65,6 +65,7 @@
         <a class="dismiss">X</a>
     </div>
     <script src="_content/DevExpress.Blazor.Resources/js/preload-script.js"></script>
+    <script src="js/typed.umd.js"></script>
     <script src="js/receiver-signature.js?v=9"></script>
     <script src="_framework/blazor.webassembly.js"></script>
 </body>