docs: added documentation for build command (#9604)

- Added documentation for `medusa build` command
- Update deployment documentation for admin to use `--admin-only` option

www/apps/resources/app/deployment/admin/vercel/page.mdx

@@ -86,7 +86,7 @@ To create a Vercel project, on your Vercel dashboard:
         - Set the Build Command field to the following:
         
         ```bash npm2yarn
-        npm run build
+        npm run build --admin-only
         ```
         
         - Set the Output Directory field to `.medusa/admin`.