Remove NavItemBlueprint in favor of page-based nav discovery

Drop the deprecated NavItemBlueprint from the public API and migrate core
plugins to set title and icon on PageBlueprint instead. AppNav keeps
backward compatibility for legacy nav-item extensions via an internal
core.nav-item.target data ref.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

docs/features/search/declarative-integration.md

@@ -52,16 +52,15 @@ _Example disabling the search page extension_
 app:
   extensions:
     - page:search: false # ✨
-    - nav-item:search: false # ✨
 ```
 
-_Example setting the search sidebar item title_
+_Example setting the search page title (used in the sidebar)_
 
 ```yaml
 # app-config.yaml
 app:
   extensions:
-    - nav-item:search: # ✨
+    - page:search: # ✨
         config:
           title: 'Search Page'
 ```