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/src/utils/get-first-category-item.ts

@@ -4,11 +4,22 @@ import {
 } from "@docusaurus/theme-common/internal"
 import { PropSidebarItem } from "@docusaurus/plugin-content-docs"
 
-export default function getFirstCategoryItem(
-  categoryLabel: string
-): PropSidebarItem | undefined {
+type Options = {
+  categoryLabel?: string
+  categoryCustomId?: string
+}
+
+export function getCategoryItems({ categoryLabel, categoryCustomId }: Options) {
   return findSidebarCategory(
     useDocsSidebar().items,
-    (item) => item.label === categoryLabel
-  )?.items[0]
+    (item) =>
+      item.label === categoryLabel ||
+      item.customProps.category_id === categoryCustomId
+  )?.items
+}
+
+export default function getFirstCategoryItem(
+  options: Options
+): PropSidebarItem | undefined {
+  return getCategoryItems(options)?.[0]
 }