[Doc] No content for building from master source

[TaoLv]

Seems this page is not up-to-date:
https://mxnet.incubator.apache.org/install/index.html&version=master#

Also, this page is a litte messy for me:
https://github.com/apache/incubator-mxnet/blob/master/docs/install/index.md

[vrakesh]

@TaoLv thank you for reporting the issue
@mxnet-label-bot [Doc]

[kalyc]

For the first document could you provide more context @TaoLv - Build from Source on Master on what machine? I do see the pages opening for building from source

[TaoLv]

@kalyc Seems it's already fixed by #12509 . I will close this issue now.