docs: add starlight-themes link (#3510)

Co-authored-by: Chris Swithinbank <[email protected]>

Author: HiDeoo

docs/src/content/docs/resources/themes.mdx

@@ -18,6 +18,8 @@ A theme is a Starlight plugin that changes the visual appearance of a site with
 
 Install a theme built by the community to quickly customize the look and feel of your site.
 
+Preview a list of all themes below or try them out interactively on the [Starlight Themes demo site](https://starlight-themes.netlify.app/).
+
 <ThemeGrid
 	labels={{
 		/** Accessible label for the theme toggle. */