[readd] diagram elements category with lower case

Author: Mc-Zen

docs/reference/Diagram elements/_category_.json

@@ -0,0 +1,8 @@
+{
+  "label": "Diagram elements",
+  "position": 1,
+  "link": {
+    "type": "generated-index",
+    "description": "Parts and elements found in a diagram. "
+  }
+}