feat: add preview feature admonition

Author: oleiade

docs/sources/k6/next/shared/preview-feature.md

@@ -0,0 +1,15 @@
+---
+title: Preview feature admonition
+---
+
+{{< admonition type="note" >}}
+
+This is a **preview** feature.
+<br>
+<br>
+While we're committed to supporting and maintaining this feature in the future, and we do not anticipate breaking changes, some functionality might be missing, a bit rough on the edges, or there might be use-cases we haven't covered yet. The feature is considered ready to use, but is in preview mode as we expect to keep iterating on it by adding new functionalities and improvements at a later point in time.
+<br>
+<br>
+Users are encouraged to participate in its roadmap and implementation on [GitHub](https://github.com/grafana/k6-jslib-testing). Feel free to [open an issue](https://github.com/grafana/k6/issues) if you have any feedback or suggestions.
+
+{{< /admonition >}}