feat: Update URL construction for add page navigation

The code changes modify the `PageController` class to update the URL construction for navigating to the add page. The `getAddPage()` method now maps to the `/add` endpoint instead of the previous `add` endpoint. This change ensures that the URL matches the correct route for the add page.

Recent commits:
- Integrate add content page into backend
- Add base URL to URL construction
- Rework URL construction

Recent repository commits:
- Integrate add content page into backend
- Merge branch 'main' into 15-make-content-adding-page-functional-js
- Merge branch '11-add-api-for-getting-home-page-data' into 'main'
- Merge branch '22-integrate-api-and-frontend' into '11-add-api-for-getting-home-page-data'
- Merge branch '11-add-api-for-getting-home-page-data' into '22-integrate-api-and-frontend'
- feature: add ability to vote on datasets
- Merge remote-tracking branch 'origin/implement-js' into 22-integrate-api-and-frontend
- Add base URL to URL construction
- Merge remote-tracking branch 'origin/implement-js' into 22-integrate-api-and-frontend
- Rework URL construction

src/main/java/de/uni_passau/fim/PADAS/group3/DataDash/controler/PageController.java

@@ -5,7 +5,7 @@ import org.springframework.web.bind.annotation.GetMapping;
 
 @Controller
 public class PageController {
-    @GetMapping("add")
+    @GetMapping("/add")
     public String getAddPage() {
         return "add";
     }    

src/main/resources/static/main.js

@@ -75,6 +75,7 @@ for (const downvoteButton of downvoteButtons) {
 // functions of the main page
 function navigateToAdd() {
     //TODO: url to add page not yet implemented, add here
+    window.location.href = "/add";
 }
 
 function filter(filterString) {