Add Dashboard variant for "Connect the GitHub application to your repository"

[stevector]

The "Connect the GitHub application to your repository" section of this page describes the appropriate Terminus command.

Many developer likely prefer the dashboard option (which didn't exist at the time this docs page was written) for connecting an existing repo.

There's a usable dashboard screenshot on https://docs.pantheon.io/nextjs/content-publisher-tutorial

We could toggle the dashboard way and the terminus way in tabs: https://docs.pantheon.io/style-guide#tabs