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/.well-known/skills/app-frontend-system-migration/SKILL.md

@@ -60,7 +60,7 @@ Even with feature discovery enabled, you can disable specific extensions via con
 app:
   extensions:
     - page:techdocs: false
-    - nav-item:search: false
+    - page:search: false
 ```
 
 ### How Discovery Works with Manual Imports

docs/.well-known/skills/mui-to-bui-migration/SKILL.md

@@ -882,8 +882,7 @@ import { CheckboxGroup, Checkbox } from '@backstage/ui';
 
 Some Backstage APIs still require MUI-compatible icon types:
 
-- **NavItemBlueprint** (`@backstage/frontend-plugin-api`): The `icon` prop expects MUI `IconComponent` type. Remix icons
-  are not type-compatible.
+- **PageBlueprint** (`@backstage/frontend-plugin-api`): The `icon` param on page extensions expects an `IconElement`. MUI icon components can still be used via `<Icon fontSize="inherit" />`.
 - **Timeline** (`@material-ui/lab`): No BUI equivalent exists.
 
 For these cases, keep using MUI components.