Add title (and anchor) for the preview section

Signed-off-by: Guillaume Tardif <guillaume.tardif@gmail.com>

Author: gtardif

docs/extensions/labels.md

@@ -1,3 +1,5 @@
+# Extension image labels
+
 [Labels](https://docs.docker.com/engine/reference/builder/#label) are specified in the extension's `Dockerfile` and are used to provide information about the extension.
 
 | Label <div style="width:170px"></div>| Required | Description | Example |
@@ -16,6 +18,8 @@
 
     If any of the previous _required_ labels are missing in the `Dockerfile`, Docker Desktop will consider the extension invalid and will not appear listed in the Marketplace.
 
+## Preview extension in marketplace
+
 You can validate that these image labels are rendered as you expect.
 
 When you build and install your unpublished extension, you can preview the extension in the Marketplace "installed" tab, and see how the extension labels are rendered in the list and in the details page of the extension.