docs(stepfunctions): update Step Functions marketplace documentation

missing latest feature in Step Functions marketplace documentation

update the Step Functions marketplace documentation to include latest features with visualization gif

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be squash-merged at release time.
- Your code changes must meet the guidelines in [CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

Author: Zelin Zhou

docs/marketplace/vscode/stepFunctions.gif

@@ -0,0 +1,4 @@
+{
+	"type": "Feature",
+	"description": "Update Step Functions marketplace documentation."
+}

packages/toolkit/.changes/next-release/Feature-7359863e-64bc-4f03-918d-3a6cea89038e.json

@@ -39,6 +39,12 @@ AWS Infrastructure Composer maintains a real-time visual representation of your
 
 Unified software development service to quickly build and deliver applications on AWS.
 
+## [Step Functions](https://aws.amazon.com/step-functions/)
+
+With the AWS toolkit, you can use Workflow Studio from within VS Code to visualize, build, and even test individual states in your state machines. You provide state inputs and set variables, start the test, then you can see how your data is transformed. You can adjust the workflow and re-test. When finished, you can apply the changes to update the state machine.
+
+![Step Functions](https://raw.githubusercontent.com/aws/aws-toolkit-vscode/HEAD/docs/marketplace/vscode/stepFunctions.gif)
+
 **Dev Environments** - launch VS Code in a cloud development environment, available on-demand in the cloud and automatically created with branch code and consistent project settings, providing faster setup, development, and testing
 
 ## View, modify, and deploy AWS resources
@@ -73,7 +79,6 @@ Unified software development service to quickly build and deliver applications o
 
 -   **EC2** - `Connect VS Code` or `Open Terminal` to EC2 instances
 -   **Redshift** - view database objects and run SQL queries in a notebook interface
--   **Step Functions** - work with asl files and render state machine visuals
 -   **CloudFormation** - view CloudFormation stacks
 -   **API Gateway** - invoke an API gateway endpoint
 -   **S3** - view and create S3 folders and buckets, download and upload files, and edit supported files