docs: add content for module 4

100-level-AsyncAPI-Intro/step3.md

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+When Chan has to go back to his planet, he wants to keep talking to Eve. Eve is sad that Chan is leaving, but she's also excited to try sending messages across space. They decide to write AsyncAPI documents to stay in touch.
+
+## Create an AsyncAPI Document
+
+Chan begins by setting up his space message plan.
+
+```yaml
+asyncapi: 3.0.0
+info:
+  title: Sending Signal to Eve
+  version: 0.1.0
+```
+
+Essentially, Chan is saying, "I'm using the `3.0.0` space message rules, and I'm naming my plan `Sending Signal to Eve.`".
+
+## Message route
+
+Next, Chan picks a path for his message. He makes a channel called `spaceMessage` with the address `Earth/letter`. 
+
+```yaml
+channels:
+  spaceMessage:
+    address: Earth/letter
+    messages:
+      lettertoEarth:
+        description: Communicating with Eve.
+        payload:
+          type: object
+          additionalProperties: false
+          properties:
+            fullName:
+              type: string
+            age:
+              type: integer
+              minimum: 18
+```
+
+## Send message
+
+Last, Chan sends the message via the `spaceMessage` channel.
+
+```yaml
+operations: 
+  spaceMessage:
+    action: send
+    channel: 
+      $ref: '#/channels/spaceMessage'
+```
+
+## Validate AsyncAPI document
+
+Chan uses [AsyncAPI Studio](https://studio.asyncapi.com/) to validate that his messages are working. Studio is a tool that checks for mistakes. He can paste his document into Studio to validate his work.
+
+## Generator
+
+Generator automatically writes code in different languages based on Chan's blueprint, saving him time and effort to focus on the real message.
+
+* Install Generator:
+
+```bash
+npm install -g @asyncapi/generator
+```
+
+* Generate code from your AsyncAPI file: 
+
+```bash
+asyncapi generate fromFile my-api.yaml
+```
+
+## Exercise 1: Chan's first AsyncAPI message
+
+1. Open the terminal and type `nano chan-to-eve.yaml` to create an AsyncAPI document for communication with Eve.
+
+2. Create the basic structure for Chan's communication:
+
+   ```yaml
+   asyncapi: 3.0.0
+   info:
+     title: Sending Signal to Eve
+     version: 0.1.0
+   channels:
+     spaceMessage:
+       address: Earth/letter
+       messages:
+         lettertoEarth:
+           description: Communicating with Eve.
+           payload:
+             type: object
+             additionalProperties: false
+             properties:
+               fullName:
+                 type: string
+               age:
+                 type: integer
+                 minimum: 18
+   ```
+
+3. Add a `spacename` property:
+
+   ```yaml
+   spacename:
+     type: string
+   ```
+
+4. Save (`Ctrl + O`, `Enter`) and exit (`Ctrl + X`).
+
+## Exercise 2: Message structure
+
+1. Open `chan-to-eve.yaml` again.
+
+2. Add `timestamp` and `priority` fields:
+
+   ```yaml
+   timestamp:
+     type: string
+     format: date-time
+   priority:
+     type: integer
+   ```
+
+3.  Save your changes (`Ctrl + O`, `Enter`, `Ctrl + X`).
+
+## Exercise 3: Charting the Message's Course
+
+1. Edit `chan-to-eve.yaml`.
+
+2. Define the sending protocol:
+
+   ```yaml
+   operations: 
+     spaceMessage:
+       action: send
+       channel: 
+         $ref: '#/channels/spaceMessage'
+   ```
+
+3. Save your changes (`Ctrl + O`, `Enter`, `Ctrl + X`).
+
+## Exercise 4: Establish communication with Capuccinova
+
+1. Create `chan-to-capuccinova.yaml` using `nano`.
+
+2. Add operations details for Capuccinova:
+
+   ```yaml
+   operations:
+     capuccinovaMessage:
+       action: send
+       channel:
+         $ref: '#/channels/capuccinovaMessage'
+   ```
+
+3. Save your work (`Ctrl + O`, `Enter`, `Ctrl + X`).