docs: Diagnose systeem documentatie

- Nieuwe diagnose-systeem.mdx met volledige gebruikersdocumentatie
- Master-detail layout, ICD-10 zoeken, status/ernst niveaus
- Categorie toegevoegd aan _index.json

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

content/nl/documentatie/_index.json

@@ -100,6 +100,14 @@
       "order": 7,
       "status": "active"
     },
+    {
+      "slug": "diagnose-systeem",
+      "title": "Diagnose Systeem",
+      "group": "features",
+      "description": "ICD-10 diagnoses vastleggen en beheren",
+      "order": 8,
+      "status": "completed"
+    },
     {
       "slug": "agenda",
       "title": "Agenda Systeem",