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`

Commencez par lire la documentation actuelle sur MatMomentDateModule et MomentDateAdapter, puis examinez le scénario de fuseau horaire décrit dans l’issue. Ajoutez un exemple de documentation montrant comment créer une date ou une date avec heure pour un fuseau horaire sélectionné manuellement, et confirmez que l’exemple correspond au comportement actuel de l’adaptateur.

Rédigé par le modèle d'indexation à partir du texte de l'issue.