docs: document custom storage drivers

docs/1.docs/8.storage.md

@@ -101,6 +101,67 @@ Then, you can use the redis storage using the `useStorage("redis")` function.
 You can find the driver list on [unstorage documentation](https://unstorage.unjs.io/) with their configuration.
 ::
 
+### Custom drivers
+
+The `driver` field accepts a built-in driver name (for example `"redis"`) or a **path** to a custom driver module. You cannot pass a driver function or object directly in config — Nitro imports drivers as separate modules so your config stays separate from runtime code.
+
+Create a driver file that **default-exports** a driver factory from `defineDriver`. To build a driver from scratch, implement the [unstorage driver interface](https://unstorage.unjs.io/guide/custom-driver) (`hasItem`, `getItem`, `setItem`, and so on):
+
+```ts [drivers/kv.ts]
+import { defineDriver } from "unstorage";
+
+export default defineDriver((opts: { prefix?: string } = {}) => {
+  const data = new Map<string, string>();
+  const prefix = opts.prefix || "";
+
+  return {
+    name: "kv",
+    options: opts,
+    hasItem(key) {
+      return data.has(prefix + key);
+    },
+    getItem(key) {
+      return data.get(prefix + key) ?? null;
+    },
+    setItem(key, value) {
+      data.set(prefix + key, value);
+    },
+    removeItem(key) {
+      data.delete(prefix + key);
+    },
+    getKeys() {
+      return [...data.keys()].map((k) =>
+        prefix ? k.slice(prefix.length) : k,
+      );
+    },
+    clear() {
+      data.clear();
+    },
+  };
+});
+```
+
+Reference it by path in your Nitro config (options besides `driver` are passed into the factory):
+
+```ts [nitro.config.ts]
+import { defineConfig } from "nitro";
+
+export default defineConfig({
+  storage: {
+    kv: {
+      driver: "./drivers/kv.ts",
+      prefix: "app:",
+    },
+  },
+});
+```
+
+Use a path relative to your project root (or an absolute path). Nitro bundles the driver for the server runtime.
+
+To **patch** an existing unstorage driver instead of writing one from scratch, wrap it inside `defineDriver`, spread the base driver, and override only the methods you need.
+
+:read-more{to="https://unstorage.unjs.io/guide/custom-driver" title="unstorage custom driver guide"}
+
 ### Development storage
 
 You can use the `devStorage` option to override storage configuration during development and prerendering.