Add version warning for those docs pages w/o version info in URL?

[wey-gu]

It seems some old docs pages are with higher ranks in Google(which is normal) [1], could we possibly add warning info to avoid user refers to them wrongly as we did in those non-latest docs with the version in URL [2]?

[1] search "nebula graph 编译", first result is: https://docs.nebula-graph.com.cn/manual-CN/3.build-develop-and-administration/1.build/1.build-source-code/
[2] https://docs.nebula-graph.com.cn/1.2.0/ with warning:

Nebula Graph 内核 2.x 已发布。 Nebula Graph 内核 1.x 与 2.x数据格式、通信协议、客户端等均双向不兼容，代码仓库不相同。2.x 请参考新版本文档。

[whitewum]

It seems some old docs pages are with higher ranks in Google(which is normal) [1], could we possibly add warning info to avoid user refers to them wrongly as we did in those non-latest docs with the version in URL [2]?

[1] search "nebula graph 编译", first result is: https://docs.nebula-graph.com.cn/manual-CN/3.build-develop-and-administration/1.build/1.build-source-code/
[2] https://docs.nebula-graph.com.cn/1.2.0/ with warning:

Nebula Graph 内核 2.x 已发布。 Nebula Graph 内核 1.x 与 2.x数据格式、通信协议、客户端等均双向不兼容，代码仓库不相同。2.x 请参考新版本文档。

大体是因为gh-pages 的sitemap文件没有更改的原因。这个事情在处理了。