re-add link to hosted sandbox (flyteorg#4856)

Signed-off-by: nikki everett <[email protected]>

docs/quickstart_guide.md

@@ -3,6 +3,22 @@
 
 In this guide, you will create and run a Flyte workflow in a local Python environment to generate the output "Hello, world!"
 
+````{dropdown} Try Flyte in your browser
+:title: text-muted
+:animate: fade-in-slide-down
+
+```{link-button} https://sandbox.union.ai/
+---
+classes: try-hosted-flyte btn-warning btn-block
+text: Hosted Flyte sandbox
+---
+```
+```{div} text-muted
+*Courtesy of [Union.ai](https://www.union.ai/)*
+```
+````
+
+
 ## Prerequisites
 
 * [Install Python 3.8x or higher](https://www.python.org/downloads/)