Angular Material Getting Started should mention ng add as primary installation option

[beeman]

Bug, feature request, or proposal:

Today I went with a beginner in Angular over adding Material Design and she started running the installation manually as she read in the getting started guide.

I think it makes most sense to have the first step in Getting Started being the ng add option.

What is the expected behavior?

Developers should be advised to use ng add @angular/material.

What is the current behavior?

Developers are currently advised to manually install the dependencies, then the snapshots, and after that the ng add option.

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

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

It should be as easy as possible for people starting with Angular to add Angular Material.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?

I would love to make the changes to the docs if the maintainers agree it should be addressed. Basically creating this issue to spark a conversation around it, if you agree I'll submit a PR 👍

[Pashozator]

Would be cool a change.

[jelbourn]

FYI @vivian-hu, @beeman is working on this himself

[Splaktar]

I don't think that a PR for this was ever submitted. This is now being worked on as part of #16426.

[Splaktar]

This was done in PR #16654.

These changes will go live on material.angular.io once 8.2.0 is published to the docs site (hopefully next week).