Merge pull request AdobeDocs#38 from AdobeDocs/COMDOX-518

Add Developer intro

Author: keharper

src/data/navigation/sections/app-development.js

@@ -1,6 +1,6 @@
 module.exports = [
     {
         title: "Overview",
-        path: "/app-development/index.md",
+        path: "/app-development/index.md"   
     }
-  ];
+];

src/pages/app-development/index.md

@@ -1,6 +1,66 @@
 ---
-title: Adobe Commerce App Developer Guide Overview
-description: 
+title: Adobe Commerce App Developer's Guide Overview
+description: Learn how to make the transition from developing Commerce PHP extensions to developing Out-of-Process apps.
 ---
 
-# Adobe Commerce App Developer Guide Overview
+# Adobe Commerce App Developer's Guide Overview
+
+Developers have traditionally used PHP to create in-process extensions that add features, functionality, services, and integrations to Adobe Commerce. This model requires any new code to be compatible with upgrades, the server's PHP version, and many other essential server applications and services that Commerce uses.
+
+Adobe Developer App Builder uses out-of-process extensibility to avoid these compatibility issues. It provides a unified third-party extensibility framework for integrating and creating custom apps that extend Adobe Commerce. Since this extensibility framework is built on Adobe's infrastructure, developers can also extend Adobe Commerce with third-party systems.
+
+## How do I port an extension into an app?
+
+Start by becoming familiar with the [App Builder documentation](https://developer.adobe.com/app-builder/docs/overview/) and [create an Adobe developer account](https://developer.adobe.com/app-builder/docs/overview/getting_access/).
+
+Once you've become comfortable with the Adobe I/O infrastructure, analyze  your current extensions and begin mapping their in-process features into the App Builder and Adobe I/O environment. Key areas to consider include:
+
+*  Frontend development
+*  APIs
+*  Plugins and observers
+*  Backend development
+*  Custom cron jobs
+*  Database data
+*  Filesystem
+
+### Frontend development
+
+[Spectrum](https://spectrum.adobe.com/page/principles/) provides all the tools you need to create the next generation of React-based applications. Adobe Commerce continues support of legacy [PWA Studio](https://developer.adobe.com/commerce/pwa-studio/) and [Luma](https://developer.adobe.com/commerce/frontend-core/) storefronts.
+
+### APIs
+
+[API Mesh for Adobe Developer App Builder](https://developer.adobe.com/graphql-mesh-gateway/) enables developers to connect multiple APIs from Adobe Commerce, other Adobe products, and 3rd party sources into a single GraphQL endpoint. An orchestration layer transforms data from these disparate sources into the formats required to perform the tasks to fulfill business and user experience requirements.
+
+### Plugins and observers
+
+[Adobe I/O Events for Adobe Commerce](https://developer.adobe.com/commerce/events/) enables building event-driven Commerce integrations using App Builder. You can define key events, like customer account updates, to be emitted from Commerce and construct apps that listen and react to these events.
+
+### Admin development
+
+The [Adobe Commerce Admin UI SDK](../admin-ui-sdk/index.md) enables an App Builder developer to extend the [Commerce Admin](https://experienceleague.adobe.com/docs/commerce-admin/start/admin/admin.html) to include custom menus and pages.
+
+### Custom cron jobs
+
+App Builder uses Apache OpenWhisk Alarms to perform the scheduling services traditionally provided by cron jobs. [Scheduling Cron Jobs with Alarms](https://developer.adobe.com/app-builder/docs/resources/cron-jobs/) walks you through the process of implementing this feature.
+
+### Database data
+
+The [Adobe I/O Key/Value Storage library](https://github.com/adobe/aio-lib-state) is an npm module that provides a JavaScript abstraction on top of distributed/cloud databases with a simple key-value store state persistence API.
+
+### Filesystem
+
+The [Adobe I/O Files library](https://github.com/adobe/aio-lib-state) provides a JavaScript abstraction on top of cloud blob storages with a simple file-system like persistence API.
+
+## Related information
+
+View the following tutorials for more information about using App Builder to build out-of-process apps:
+
+*  [Introduction to App Builder](https://experienceleague.adobe.com/docs/commerce-learn/tutorials/adobe-developer-app-builder/introduction-to-app-builder.html)
+
+*  [Extensibility Framework for App Builder](https://experienceleague.adobe.com/docs/commerce-learn/tutorials/adobe-developer-app-builder/extensibility-framework-commerce-eventing.html)
+
+*  [Functional demonstration for App Builder](https://experienceleague.adobe.com/docs/commerce-learn/tutorials/adobe-developer-app-builder/app-builder-functional-demonstration.html)
+
+*  [Build your first App Builder app](https://experienceleague.adobe.com/docs/commerce-learn/tutorials/adobe-developer-app-builder/first-app/overview.html)
+
+*  [Get started with API Mesh](https://experienceleague.adobe.com/docs/commerce-learn/tutorials/adobe-developer-app-builder/api-mesh/getting-started-api-mesh.html)