[Documentation] Move Venia sample data installation into its own topic

[jcalcaben]

This issue is for the following packages:

[ ] venia-concept
[ ] pwa-buildpack
[ ] peregrine
[x] pwa-devdocs
[ ] upward-js
[ ] upward-spec

This issue is a:

[ ] Bug
[ ] Feature suggestion
[x] Documentation issue
[ ] Other (Please Specify)

Description

Currently the Venia setup instructions contain a section about installing the Venia sample data into a local instance of Magento. Since the UPWARD implementation, having a running instance of Magento is optional and makes instructions for installing the sample data seem out of place.

Expected result:

A new topic that is focused on installing the Venia sample data into Magento.

Possible solutions:

Create a new doc topic and move the sample data installation instructions over from the Setup topic.

• Include prerequisites, including mention of PHP 7.2 incompatibility on one of the modules. Must use 7.1.
• Revise instructions so that it has the user copying the bash script from the venia project instead of showing the content for the bash script.