docs: improvements to deployment guides (#6183)

- Add a new deployment overview page giving a general overview of how a Medusa project is deployed
- Add a new section in all backend deployment guides related to the Medusa admin.
- Add a general deployment guide for the medusa admin.
- Add a general deployment guide for the Next.js starter

www/apps/docs/content/upgrade-guides/index.mdx

@@ -13,23 +13,31 @@ Find in this page the upgrade guides that require necessary steps when upgrading
 ## Backend
 
 <DocCard
-  item={getFirstCategoryItem('Backend')}
+  item={getFirstCategoryItem({
+    categoryLabel: 'Backend'
+  })}
 />
 
 ## Admin Dashboard
 
 <DocCard
-  item={getFirstCategoryItem('Admin Dashboard')}
+  item={getFirstCategoryItem({
+    categoryLabel: 'Admin Dashboard'
+  })}
 />
 
 ## Medusa React
 
 <DocCard
-  item={getFirstCategoryItem('Medusa React')}
+  item={getFirstCategoryItem({
+    categoryLabel: 'Medusa React'
+  })}
 />
 
 ## Medusa UI
 
 <DocCard
-  item={getFirstCategoryItem('Medusa UI')}
+  item={getFirstCategoryItem({
+    categoryLabel: 'Medusa UI'
+  })}
 />