docs: mocking (#270)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Added a new sidebar entry under "Advanced" for quick access to
additional guidance.
- **Documentation**
- Introduced a new "Mocking" guide detailing approaches for creating
alternative implementations for testing.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: dinwwwh

apps/content/.vitepress/config.ts

@@ -147,6 +147,7 @@ export default defineConfig({
             { text: 'Validation Errors', link: '/docs/advanced/validation-errors' },
             { text: 'RPC Protocol', link: '/docs/advanced/rpc-protocol' },
             { text: 'RPC JSON Serializer', link: '/docs/advanced/rpc-json-serializer' },
+            { text: 'Mocking', link: '/docs/advanced/mocking' },
             { text: 'Exceeds the maximum length ...', link: '/docs/advanced/exceeds-the-maximum-length-problem' },
           ],
         },

apps/content/docs/advanced/mocking.md

@@ -0,0 +1,30 @@
+---
+title: Mocking
+description: Easily mock your oRPC handlers for testing.
+---
+
+# Mocking
+
+Mock your oRPC handlers with ease.
+
+::: warning
+This page is incomplete and may be missing important information.
+:::
+
+## Using the Implementer
+
+The [Implementer](/docs/contract-first/implement-contract#the-implementer) is designed for contract-first development. However, it can also be used to create alternative versions of your [router](/docs/router) or [procedure](/docs/procedure) for testing purposes.
+
+```ts twoslash
+import { router } from './shared/planet'
+// ---cut---
+import { implement, unlazyRouter } from '@orpc/server'
+
+const fakeListPlanet = implement(router.planet.list).handler(() => [])
+```
+
+You can now use `fakeListPlanet` to replace `listPlanet`. Additionally, the `implement` function can be used to create a fake server for front-end testing.
+
+::: warning
+The `implement` function does not support the [lazy router](/docs/router#lazy-router) yet. Please use the `unlazyRouter` utility to convert your lazy router before implementing.
+:::