Link to list of themes from themes docs

docs/content/documentation/themes/installing-and-using-themes.md

@@ -18,6 +18,8 @@ Cloning the repository using Git or another VCS will allow you to easily
 update it but you can also simply download the files manually and paste
 them in a folder.
 
+You can find a list of themes [on this very website](./themes/_index.md).
+
 ## Using a theme
 
 Now that you have the theme in your `themes` directory, you only need to tell

docs/content/documentation/themes/overview.md

@@ -8,3 +8,5 @@ but still easy to update if needed.
 
 All themes can use the full power of Gutenberg, from shortcodes to Sass compilation.
 
+A list of themes is available [on this very website](./themes/_index.md).
+