docs: re-add AsyncAPI Documents and change code snippet (#2274)

Co-authored-by: Alejandra Quetzalli <[email protected]>

pages/docs/tutorials/getting-started/asyncapi-documents.md

@@ -0,0 +1,46 @@
+---
+title: "AsyncAPI documents"
+date: 2019-04-01T10:56:52+01:00
+menu:
+  docs:
+    parent: 'getting-started'
+weight: 101
+---
+
+An AsyncAPI document is a file that defines and annotates the different components of **a specific Event-Driven API**.
+
+The file format must be JSON or YAML; however, only the subset of YAML that matches the JSON capabilities is allowed.
+
+<CodeBlock>
+{`asyncapi: 3.0.0
+info:
+  title: Example application
+  version: '0.1.0'
+channels:
+  userSignup:
+    address: 'user/signedup'
+    messages:
+      userSignedupMessage: 
+        description: A message describing that a user just signed up.
+        payload:
+          type: object
+          additionalProperties: false
+          properties:
+            fullName:
+              type: string
+            email:
+              type: string
+              format: email
+            age:
+              type: integer
+              minimum: 18
+operations:
+  publishUserSignedup:
+    action: 'send'
+    channel:
+      $ref: '#/channels/userSignup'`}
+</CodeBlock>
+
+The AsyncAPI document is a machine-readable definition of your Event-Driven API. That document can be used afterward to generate documentation and code, validate the messages that `Example application` sends, and even apply API management policies to your messages before they arrive at the broker.
+
+Your API documentation is now machine-readable (easily parseable by code) so the myriad of useful applications is endless.