docs(layout): document print classes and their usage

[Splaktar]

Bug, enhancement request, or proposal:

Proposal

CodePen and steps to reproduce the issue:

CodePen Demo which demonstrates the issue:

https://material.angularjs.org/latest/layout/introduction

Detailed Reproduction Steps:

1. Search the docs for guidance on styling for printing

What is the expected behavior?

Docs would be found which include an explanation about the ability to use classes like show-print, hide-print, show-print-xs, and hide-print-lg, etc.

What is the current behavior?

No documentation for this feature exists.

What is the use-case or motivation for changing an existing behavior?

The feature is unknown to many and questions are raised when the docs can't be found.

Which versions of AngularJS, Material, OS, and browsers are affected?

• AngularJS: 1.6.7
• AngularJS Material: 1.1.0+
• OS: All
• Browsers: All

Is there anything else we should know? Stack Traces, Screenshots, etc.

Printing styling was greatly improved in https://github.com/angular/material/pull/6923. The main commit to look at for updating the layout docs is https://github.com/angular/material/pull/6923/commits/eb1249da5c4db0f352370227e3dd1cc84efebb74.