Browse to github.com/PowerShell/PowerShell
Scroll down to the "Get PowerShell" section
Select any of the "Instructions" links
Page appears telling how to install PowerShell
404
This is not the web page you are looking for.
Occurs using Chrome on Windows 10
Occurs using Firefox on Ubuntu 16.04 LTS
It looks like a recent commit moved the docs into it's own repo, and I'm wondering if the /installation/. folder got missed somehow.

The "Instructions" was removed by #6899.
Most helpful comment
The "Instructions" was removed by #6899.
6981 has been requested to fix this.