Installation instructions nightly test failure

[mseth10]

Description

PR #15885 removed file docs/install/index.md which is used by nightly_test_installation here, causing nightly tests to fail since 09/19.

Here's the jenkins link for the failed job:
http://jenkins.mxnet-ci.amazon-ml.com/blue/organizations/jenkins/NightlyTests/detail/master/471/pipeline/

[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 label(s): Test

[mseth10]

@mxnet-label-bot add[Doc, Test]

[mseth10]

@aaronmarkham since this is caused by your PR, can you please take a look? Thanks!

[aaronmarkham]

Here's an example of where the install instructions reside:
https://github.com/apache/incubator-mxnet/blob/master/docs/static_site/src/_includes/get_started/macos/python/cpu/pip.md

There are many .md files now that are specific to the platform or kind of installation you want. That nightly test needs to be updated for the new pages.

I think it would make more sense to have the commands as shell/text file includes that can be executed directly as well as included using the Jekyll include syntax {% include /get_started/pip_snippet.md %} to appear in the instructions.