docs: add *.example.json onboarding files for all three hosts

DeepDrftContent/apikey.example.json

@@ -0,0 +1,5 @@
+{
+  "ApiKeySettings": {
+    "ApiKey": "your-secret-api-key-here"
+  }
+}

DeepDrftContent/filedatabase.example.json

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+{
+  "FileDatabaseSettings": {
+    "VaultPath": "C:/Development/DeepDrftHome/Database/Vaults"
+  }
+}