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.
+
+```
+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.
+
+
+```
+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.
+
+## 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`).
+
+## Test your work
+
+After each exercise, use **AsyncAPI Studio** to validate Chan's YAML documents. After all, you want to ensure seamless communication with Eve and the new connection to Capuccinova! Thanks to you, Eve and Chan can now keep their friendship alive by communicating across galaxies.