Refactor macOS install instructions
What does this MR do?
Ensures it's clear which is the official installation method for GitLab Runner on MacOS X.
- Before: https://docs.gitlab.com/runner/install/osx.html
- After: http://review-docs-docs-b96uoe.178.62.207.141.xip.io/runner/install/osx.html
Closes #4136 (closed).
Edited by Evan Read