Added DDD page

docs/en/Domain-Driven-Design.md

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+# Domain Driven Design
+
+## What is DDD?
+
+ABP framework provides an **infrastructure** to make **DDD** based development easier to implement. DDD is [defined in the Wikipedia](https://en.wikipedia.org/wiki/Domain-driven_design) as below:
+
+> **Domain-driven design** (**DDD**) is an approach to software development for complex needs by connecting the implementation to an evolving model. The premise of domain-driven design is the following:
+>
+> - Placing the project's primary focus on the core domain and domain logic;
+> - Basing complex designs on a model of the domain;
+> - Initiating a creative collaboration between technical and domain experts to iteratively refine a conceptual model that addresses particular domain problems.
+
+### Layers
+
+ABP follows DDD principles and patterns to achieve a layered application model which consists of four fundamental layers:
+
+- **Presentation Layer**: Provides an interface to the user. Uses the *Application Layer* to achieve user interactions.
+- **Application Layer**: Mediates between the Presentation and Domain Layers. Orchestrates business objects to perform specific application tasks. Implements use cases as the application logic.
+- **Domain Layer**: Includes business objects and their business rules. This is the heart of the application.
+- **Infrastructure Layer**: Provides generic technical capabilities that support higher layers mostly using 3rd-party libraries.
+
+## Contents
+
+* **Domain Layer**
+  * [Entities & Aggregate Roots](Entities.md)
+  * Value Objects
+  * [Repositories](Repositories.md)
+  * Domain Services
+  * Specifications
+* **Application Layer**
+  * [Application Services](Application-Services.md)
+  * [Data Transfer Objects (DTOs)](Data-Transfer-Objects.md)
+  * Unit of Work

docs/en/Unit-Of-Work.md

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+## Unit of Work
+
+TODO

docs/en/docs-nav.json

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     },
     {
       "text": "Domain Driven Design",
+      "path": "Domain-Driven-Design.md",
       "items": [
         {
           "text": "Domain Layer",
@@ -170,7 +171,8 @@
           "text": "Application Layer",
           "items": [
             {
-              "text": "Application Services"
+              "text": "Application Services",
+              "path": "Application-Services.md"
             },
             {
               "text": "Data Transfer Objects"