Adding info to Style Guide about default behaviour of Details component. (#17674)

Author: Oxyjun

src/content/docs/style-guide/components/details.mdx

@@ -10,4 +10,14 @@ import { Details } from "~/components"
 <Details header="Open me!">
     Hello, world!
 </Details>
-```
+```
+
+You can specify the default configuration of each instance of the `<Details>` component (that is, whether it is open or closed by default).
+
+```mdx live
+import { Details } from "~/components"
+
+<Details header="Close me!" open = {true}>
+    Long piece of code example.
+</Details>
+```