Add ability to specify sub categories in sidebar.json (#891)

docs/guides-navigation.md

@@ -94,6 +94,29 @@ You should provide `directory/id` instead of `id` in `sidebars.json`.
 }
 ```
 
+### Adding Sub Categories 
+
+It is possibile to add sub categories to a sidebar. Instead of passing an array to the category like the previous examples you can pass an object where 
+the keys will be the sub category name. You can then pass an array of document ids to the sub category.
+
+```js
+{
+  "examples-sidebar" : {
+    "My Example Category" : {
+      "My Example Sub Category" : [
+        "my-examples",
+        ...
+      ],
+      "My Next Sub Category" : [
+        "some-other-examples"
+      ]
+      ...
+    },
+    ...
+  }
+}
+```
+
 ### Adding New Sidebars
 
 You can also put a document in a new sidebar. In the following example, we are creating an `examples-sidebar` sidebar within `sidebars.json` that has a category called `My Example Category` containing a document with an `id` of `my-examples`.