docs(backend): Add indexes into i18n column

docs/pages/backend/i18n.mdx

@@ -34,23 +34,35 @@ The translation table must have:
 - any fields for relation with main table (in our case `nav_id`)
 
 ```ts
-export const core_nav_name = pgTable("core_nav_name", {
-  id: serial("id").primaryKey(),
-  nav_id: serial("nav_id")
-    .notNull()
-    .references(() => core_nav.id, {
-      onDelete: "cascade"
-    }),
-  language_code: varchar("language_code")
-    .notNull()
-    .references(() => core_languages.code, {
-      onDelete: "cascade"
-    }),
-  value: varchar("value", { length: 50 }).notNull()
-});
+import { relations } from "drizzle-orm";
+import { index, pgTable, serial, varchar } from "drizzle-orm/pg-core";
+
+export const core_nav_name = pgTable(
+  "core_nav_name",
+  {
+    id: serial("id").primaryKey(),
+    nav_id: serial("nav_id")
+      .notNull()
+      .references(() => core_nav.id, {
+        onDelete: "cascade"
+      }),
+    language_code: varchar("language_code")
+      .notNull()
+      .references(() => core_languages.code, {
+        onDelete: "cascade"
+      }),
+    value: varchar("value", { length: 50 }).notNull()
+  },
+  table => ({
+    nav_id_idx: index("core_nav_name_nav_id_idx").on(table.nav_id),
+    language_code_idx: index("core_nav_name_language_code_idx").on(
+      table.language_code
+    )
+  })
+);
 ```
 
-Remember to set `onDelete: 'cascade'` action into references. We want to delete translation when we delete main table.
+Remember to set `onDelete: 'cascade'` action into references and set `indexes` for best performerce. We want to delete translation when we delete main table.
 
 ### Relation