新增 Elasticsearch 笔记 - #1564

学习站：内容与 learn/content.lock.json 不一致，请在本机运行 python3 learn/maintain.py 查看并写入：{"added": {"questions": 0, "days": 0, "routes": 0}, "edited": {"questions": 32, "days": 5}, "retired": {"questions": 2, "days": 0, "routes": 0}}