Refactor service and script paths for LightRAG.

- Renamed service file to `lightrag-server.service.example`
- Updated `ExecStart` path in service file
- Corrected README.md service file reference
- Moved `start_lightrag.sh.example` to root
- No functionality is changed

lightrag-server.service.example

@@ -0,0 +1,17 @@
+[Unit]
+Description=LightRAG XYJ Ollama Service
+After=network.target
+
+[Service]
+Type=simple
+User=netman
+# Memory settings
+MemoryHigh=8G
+MemoryMax=12G
+WorkingDirectory=/home/netman/lightrag-xyj
+ExecStart=/home/netman/lightrag-xyj/start_lightrag_server.sh
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target