Refine quick start: Update images (microsoft#328)

# Description

- Update the image to use `pf` env which is specified by previous steps
- Update flow editor test gif

# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes]**
- [x] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [x] **I have read the [contribution guidelines](../CONTRIBUTING.md).**

## General Guidelines and Best Practices
- [x] Title of the pull request is clear and informative.
- [x] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

docs/how-to-guides/quick-start.md

@@ -273,7 +273,7 @@ Use the code lens action on the top of the yaml editor to trigger flow test
 
 
 Click the run flow button on the top of the visual editor to trigger flow test.
-![visual_editor_flow_test](../media/how-to-guides/quick-start/run_flow_visual_editor.png)
+![visual_editor_flow_test](../media/how-to-guides/quick-start/test_flow_dag_editor.gif)
 :::
 
 ::::