Fix playground docs (#6415)
Fix playground docs (#6415)

Fixes #6414

Adds missing `pod install` to  playground installation instructions at `website/docs/docs-playground-app.mdx`:

```js
1. Install dependencies via `npm install` (if you haven't already) and `cd ./playground/ios && pod install` (for iOS)
```
wix/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation