Supported versions of PHP not provided

[jms-pantheon]

Re: PHP on Pantheon

Also: Pantheon YAML Configuration

Priority: Low

Issue Description:

Right now we have instructions in two places on setting the php_version but we don't provide the available options, so users have to trial and error to match up.

The information does exist in the introduction section of the guide page: https://docs.pantheon.io/guides/php

Suggested Resolution

Condense the supported versions into a reusable component and include wherever instructions are provided