docs: add documentation on all shortcodes (#753)

layouts/_shortcodes/filetree/container.html

@@ -1,3 +1,9 @@
+{{- /*
+A file tree container.
+
+@example {{< filetree/container >}}{{< /filetree/container >}}
+*/ -}}
+
 <div class="hextra-filetree hx:mt-6 hx:select-none hx:text-sm hx:text-gray-800 hx:dark:text-gray-300 not-prose">
   <div class="hx:inline-block hx:rounded-lg hx:px-4 hx:py-2 hx:border hx:border-gray-200 hx:dark:border-neutral-800">
     {{- .InnerDeindent -}}