Install and setup: Instructions on how to work with Pantheon

Problem/Motivation

We (and other agencies) mostly work with Pantheon. So we should include instructions on how to get your brand new Sous project running on Pantheon.

Proposed resolution

Add to the install documentation for things like:
* Creating a Pantheon site with an empty upstream
* Adding its git repo (see #3407117: Install and setup: Rethink how to set up stuff with Git → )
* Adding the `pantheon.yml` file
* Modifying .lando.yml to work with Pantheon
* Adding the `web/sites/default/*panth*` stuff and include in `settings.php`
* Anything else required???

Remaining tasks