[docs] Update Adding a new module to an existing application section in Expo Modules API: Get started

[amandeepmittal]

Why

Context Slack

How

• Add context to use Prebuild command to generate native project directories and use npx expo run commands to compile and run the example app instead of using EAS Build.
• Update Android and iOS instructions in step 3
• Update instructions in Step 2 to provide an example how to use the method from the native module

Test Plan

Run docs locally and see: http://localhost:3002/modules/get-started/#adding-a-new-module-to-an-existing-application or see Preview link below.

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[expo-bot]

📘 Your docs preview website is ready!

[tsapeta]

Great, thank you for improving it! I left a couple of comments.

I'm also wondering if recommending building/running the native project from Xcode and Android Studio wouldn't be better than npx expo run and re-running it every time you change something.

User could just run npx expo start once and then use the native editors which I believe offer a bit better DX for native code development. You don't need to switch between the editor and terminal, you have access to the native logs, debugger etc.

[tsapeta]

It's not necessary to run prebuild after each native change. You only need to run it again.

pod install might be necessary when you modify expo-module.config.json (e.g. add another module class) and when you add more files to the module (they won't appear in Xcode if you add them using something else than Xcode).

[amandeepmittal]

Suggested change

	For example, change the `hello` method to return a different string such as "Hello world! 🌎🤖". Then, run `npx expo prebuild` to generate the native directory and use `npx expo:android` to run the example app.
	For example, change the `hello` method to return a different string such as "Hello world! 🌎🤖". Then, run `npx expo prebuild` to generate the native directory and use `npx expo:android` to run the example app.

It's not necessary to run prebuild after each native change. You only need to run it again.

@tsapeta, does the comment mean, we don't need to run npx expo prebuild at this step too? (similarly for iOS below).