[DOC] Build from source link for ubuntu is broken

[apeforest]

On this page: https://github.com/apache/incubator-mxnet/blob/master/docs/install/build_from_source.md

If click the ubuntu hyperlink, it is broken:
https://github.com/apache/incubator-mxnet/blob/master/docs/install/ubuntu_setup.html

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended labels: Build

[vrakesh]

Looking into this, this seems to be true for all the links. since we convert our md docs to html for the docs website, looks like html links are hardcoded