chore: add CONTRIBUTING.md and describe demo apps in docs

Author: artus9033

CONTRIBUTING.md

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+# Contributing to React Native Brownfield
+
+## Project setup
+
+Run `yarn` in the root of the repository to install all dependencies.
+
+Depending on your needs, you may need to install CocoaPods in a subset of the below directories:
+- example React Native iOS app: `cd apps/RNApp/ios && pod install`
+- integrated iOS tester app: `cd apps/tester-integrated/swift && pod install`
+
+## Scripts
+
+- `lint` - runs linting on all JS/TS source files in the monorepo *[Turbo]* 
+- `typecheck` - runs TypeScript type checking on all TS source files in the monorepo *[Turbo]* 
+- `build` - runs all `build*` tasks in the Turbo repo - see below for more details *[Turbo]* 
+- `release` - releases a new version of React Native Brownfield package using `release-it`
+- `brownfield:plugin:publish:local` - publishes the Brownfield Gradle plugin to your local Maven repository for testing purposes
+- `build:brownfield` - builds the React Native Brownfield package (`packages/react-native-brownfield`) *[Turbo]* 
+- `build:docs` - builds the documentation site (`docs/`) *[Turbo]* 
+- `build:tester-integrated:android` - builds the Android integrated tester app (`apps/tester-integrated/android`) *[Turbo]* 
+- `build:tester-integrated:ios` - builds the iOS integrated tester app (`apps/tester-integrated/swift`) *[Turbo]* 
+- `build:example:android-rn` - builds the example React Native app for Android (`apps/RNApp/android`) *[Turbo]* 
+- `build:example:ios-rn` - builds the example React Native app for iOS (`apps/RNApp/ios`) *[Turbo]* 
+- `build:example:android-consumer` - builds the example native Android consumer app (`apps/AndroidApp`) *[Turbo]* 
+- `build:example:ios-consumer` - builds the example native iOS consumer app (`apps/iOSApp`) *[Turbo]* 

docs/docs/docs/getting-started/_meta.json

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-["introduction", "quick-start"]
+["introduction", "quick-start", "examples"]

docs/docs/docs/getting-started/examples.mdx

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+# Examples
+
+You can find example projects demonstrating the usage of React Native Brownfield in the [apps](https://github.com/callstack/react-native-brownfield/tree/master/apps) directory.
+
+It contains the following apps:
+
+- `RNApp` - a React Native app that is packaged using React Native Brownfield to an artifact consumed in native apps, in which you can refer to an exemplar usage of the library for packaging a React Native app to a consumable artifact
+- `AndroidApp` - an Android app consuming the packaged artifact from `RNApp`, in which you can refer to an exemplar usage of the library for integrating React Native Brownfield into a native Android app
+- `iOSApp` - an iOS app consuming the packaged artifact from `RNApp`, in which you can refer to an exemplar usage of the library for integrating React Native Brownfield into a native iOS app
+- `tester-integrated` - a native app (iOS and Android) with React Native Brownfield integrated directly into the native project without the intermediate step of packaging an artifact; this app serves mostly internal repo testing purposes