Docs: fix doc site copyright to current year (#3741)

docs/conf.py

 import os
 import sys
+from datetime import datetime
 
 sys.path.insert(0, os.path.abspath("../.."))
 
@@ -9,7 +10,7 @@ with open(version_file, "r") as f:
 __version__ = locals()["__version__"]
 
 project = "SGLang"
-copyright = "2023-2024, SGLang"
+copyright = f"2023-{datetime.now().year}, SGLang"
 author = "SGLang Team"
 
 version = __version__