docs: add new consumer concept doc

pages/docs/concepts/consumer.md

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+---
+title: Consumer
+weight: 4
+---
+
+## What is a consumer?
+In an Event Driven Architecture (EDA), a consumer is an application that listens for a particular event from a broker and reacts to it.
+
+## Why do we need consumers?
+Unlike traditional REST APIs, in EDA, event consumers are not expected to respond immediately on the same connection. In this architecture, a consumer is unaware of the producer or other consumers; all they know is that when a broker sends them an event, it is because they subscribed to it.
+
+When you want events processed asynchronously in your application, the consumer plays an important role in completing that event data flow in the event channel.
+
+```mermaid
+flowchart LR
+    a[Producer] -- Event A --->c[(Broker)]
+    a -- Event B ---> c
+    c -- Event A ---> d[Consumer A]
+    c -- Event A ---> e[Consumer B]
+    c -- Event B ---> e[Consumer B]
+    c -- Event B ---> f[Consumer C]
+    subgraph Consumers
+    d
+    e
+    f
+    end
+```
+The above diagram depicts a sample flow of events from `producer` to `broker` to `consumer`. In this instance, the `producer` publishes two events _(A and B)_ and sends them to the `broker`. Then each `consumer` subscribes to receive those events.
+
+<Remember>
+<b>Subscribers</b> can also be <a href="https://www.asyncapi.com/docs/concepts/producer">producers</a>.
+</Remember>