Update extensions.md

Since 'Preview your guide' had its own bullet point in the ##Overview section, I thought it deserved its own L2 heading.

docs/documentation/extensions.md

@@ -86,6 +86,8 @@ With this option you have a very small README file and a separate MkDocs guide w
 
 Add some [markdown](/documentation/markdown.md) content in `docs/index.md`.
 
+## Preview your guide
+
 Now you should be able to run `mkdocs serve` from within your project directory to start previewing your content. See [instructions here](/documentation/index.md#mkdocs) to get MkDocs set up.
 
 You can add more pages by creating more markdown files and specifying these files under `pages` in `mkdocs.yml`.