Use category instead of place holder markdown file as directory (#299)

Reviewed-on: https://gitea.com/gitea/docs/pulls/299

docs/usage/_category_.json

@@ -0,0 +1,9 @@
+{
+  "label": "Usage",
+  "position": 35,
+  "link": {
+    "type": "generated-index",
+    "slug": "/usage",
+    "title": "Usage"
+  }
+}