docs: new concept doc (application)

pages/docs/concepts/application.md

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+---
+title: Application
+weight: 20
+---
+
+
+## What is an application?
+An application is any computer program or a group of them. 
+
+An application could also be a micro-service, IoT device (sensor), mainframe process, etc. Users may even write applications in different programming languages if they support one of the selected protocols. 
+
+## Why do we need applications?
+In Event-Driven Architecture (EDA), an application must be a `producer`, a `consumer`, or both. Applications must also use the protocols the server supports if they wish to connect and exchange messages.
+
+### Applications: producers and consumers
+```mermaid
+flowchart TD
+        A[PRODUCER application] --> B[message] 
+        B --> C[channel] 
+        C --> D[message] 
+        D --> F[CONSUMER application]
+```
+The above diagram describes a message communication traveling through a channel between a **PRODUCER application** and a **CONSUMER application**. 
+
+<Remember>
+When writing your AsyncAPI document, make sure to describe what a user can do with your application; not what the application does. In other words, if your <em>application</em> is a <b>producer</b>, your AsyncAPI document should describe where users can subscribe to, to receive messages produced by your <b>producer</b> application. 
+</Remember>