dx required `ANDROID_NDK_HOME` while it's not mentioned in doc


The error says:

> Build failed: Other(Android linker not found. Please set the `ANDROID_NDK_HOME` environment variable to the root of your NDK installation.)

However, in the [Mobile App - Android](https://dioxuslabs.com/learn/0.6/guides/mobile/#android) chapter, it only gives:

```
export JAVA_HOME="/Applications/Android Studio.app/Contents/jbr/Contents/Home"
export ANDROID_HOME="$HOME/Library/Android/sdk"
export NDK_HOME="$ANDROID_HOME/ndk/25.2.9519653"
export PATH="$PATH:$ANDROID_HOME/emulator:$ANDROID_HOME/platform-tools"
```

So `ANDROID_NDK_HOME` should also be added into the env list.

Open the Mobile App - Android chapter linked in the issue and compare its environment-variable example with the reported linker error. Update the environment list to include ANDROID_NDK_HOME, then verify the example documents the variable named by the error alongside the existing Android paths.

Written by the indexing model from the issue text.