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

start_lightrag.sh.example

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+source /home/netman/lightrag-xyj/venv/bin/activate
+lightrag-server