consolidate duplicate preferences documentation pages

- Remove /docs/products/auth/preferences-storage (Journeys page)
- Update /docs/products/auth/preferences (Concepts page) with localStorage section
- Add redirect from old URL to prevent broken links
- Remove preferences-storage from navigation sidebar

Author: ebenezerdon

src/redirects.json

@@ -767,5 +767,9 @@
     {
         "link": "/products/sites/offer-300-s",
         "redirect": "/products/sites/offer-300?format=short"
+    },
+    {
+        "link": "/docs/products/auth/preferences-storage",
+        "redirect": "/docs/products/auth/preferences"
     }
 ]

src/routes/docs/products/auth/+layout.svelte

@@ -116,10 +116,6 @@
                 {
                     label: 'Multi-tenancy',
                     href: '/docs/products/auth/multi-tenancy'
-                },
-                {
-                    label: 'Preferences storage',
-                    href: '/docs/products/auth/preferences-storage'
                 }
             ]
         },