Add docs example of custom timezone with MomentDateAdapter

#### Bug, feature request, or proposal:
[X] Feature Request

#### What is the expected behavior?
MomentDateAdapter should have timezone support.
Moment has a timezone lib (http://momentjs.com/timezone/) which you can set the timezone
manually. This is needed when you want to create a common date/datetime based in a specific timezone.

#### What is the current behavior?
MomentDateAdapter supports browser's timezone only when creating a new date moment object.

#### What are the steps to reproduce?
* Include MatMomentDateModule to use MomentDateAdapter
* Create a mat-datepicker
* Console the output (date generated)

#### What is the use-case or motivation for changing an existing behavior?
I do want to create a common date/datetime based in a specific timezone.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular CLI: 1.5.0
Node: 8.8.1
OS: darwin x64
Angular: 5.0.1
... animations, common, compiler, compiler-cli, core, forms
... http, platform-browser, platform-browser-dynamic
... platform-server, router

`@angular/cdk: 5.0.0-rc0`
`@angular/cli: 1.5.0`
`@angular/flex-layout: 2.0.0-beta.10-4905443`
`@angular/material-moment-adapter: 5.0.0-rc0`
`@angular/material: 5.0.0-rc0`
`@angular-devkit/build-optimizer: 0.0.32`
`@angular-devkit/core: 0.0.20`
`@angular-devkit/schematics: 0.0.34`
`@ngtools/json-schema: 1.1.0`
`@ngtools/webpack: 1.8.0`
`@schematics/angular: 0.1.2`
`typescript: 2.4.2`
`webpack-sources: 1.0.1`
`webpack: 3.8.1`

首先阅读有关 MatMomentDateModule 和 MomentDateAdapter 的当前文档，然后查看 issue 中描述的时区场景。添加一个文档示例，展示如何为手动选择的时区创建日期或日期时间，并确认该示例与适配器的当前行为一致。

由索引模型根据 Issue 内容生成。