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).

CC @nolith @sirbjoern @remcohaszing @axil