Add content to async validation guide on website

website/src/routes/guides/(advanced)/async-validation/index.mdx

@@ -1,9 +1,48 @@
 ---
 title: Async validation
+description: >-
+  By default, I validate each schema synchronously. This is usually the fastest
+  way to validate unknown data, but sometimes you need to validate something
+  asynchronously.
 contributors:
   - fabian-hiller
 ---
 
+import { Link } from '@builder.io/qwik-city';
+
 # Async validation
 
-> The content of this page is not yet ready. Check back in a few days.
+By default, I validate each schema synchronously. This is usually the fastest way to validate unknown data, but sometimes you need to validate something asynchronously. For example, you might want to check if a username already exists in your database.
+
+## How it works
+
+To be able to do this, I provide an asynchronous implementation for almost every synchronous implementation. The only difference is that the asynchronous implementation always returns a promise. Otherwise, the API is exactly the same.
+
+### Naming
+
+The asynchronous implementation starts with the same name as the synchronous one, but adds the suffix `Async` to the end. For example, the asynchronous implementation of <Link href="/api/string/">`string`</Link> is called <Link href="/api/stringAsync/">`stringAsync`</Link> and the asynchronous implementation of <Link href="/api/object/">`object`</Link> is called <Link href="/api/objectAsync/">`objectAsync`</Link>.
+
+### Nesting
+
+Asynchronous functions can only be nested inside other asynchronous functions. This means that if you need to validate a string within an object asynchronously, you must also switch the object validation to the asynchronous implementation.
+
+This is not necessary in the other direction. You can nest synchronous functions within asynchronous functions, and we recommend that you do so in most cases to keep complexity and bundle size to a minimum.
+
+#### Rule of thumb
+
+We recommend that you always start with the synchronous implementation, and only move the necessary parts to the asynchronous implementation as needed. If you are using TypeScript, it is not possible to make a mistake here, as our API is completely type-safe and will notify you when you embed an asynchronous function into a synchronous function.
+
+### Example
+
+Let's say you want to validate a profile object and the username should be checked asynchronously against your database. Only the object and username validation needs to be asynchronous, the rest can stay synchronous.
+
+```ts
+import * as v from 'valibot';
+import { isUsernameAvailable } from '~/api';
+
+const ProfileSchema = v.objectAsync({
+  username: v.stringAsync([v.customAsync(isUsernameAvailable)]),
+  avatar: v.string([v.url()]),
+  description: v.string([v.maxLength(1000)]),
+});
+```

website/src/routes/guides/(schemas)/other/index.mdx

@@ -1,5 +1,8 @@
 ---
 title: Other
+description: >-
+  This guide explains other special schema functions such as literal, instance,
+  special and recursive that are not covered in the other guides.
 contributors:
   - fabian-hiller
 ---