bug(material/datepicker): Re-evaluate how the MatDatepicker handles timezones

#### Bug, feature request, or proposal:
Documentation request

#### What is the expected behavior?
The moment object returned by selecting a date with Datepicker should have no timezone information (or UTC)

#### What is the current behavior?
Currently the moment object looks like this: Wed Apr 18 2018 00:00:00 GMT+0300
(The user is located in GMT+3, so his timezone is used)

#### What is the use-case or motivation for changing an existing behavior?
I wonder how to make my matDatepicker return UTC dates so that they are independent of the timezone the user is currently in. I've found lots of issues regarding this (https://github.com/angular/material2/issues/7167), but no clear and simple explanation on how to easily achieve that.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 5.2.0, Angular Material 6.0.0-beta.4, Typescript 2.5.3, ANY browser (not just IE!)

#### Is there anything else we should know?
I'm importing MatMomentDateModule, so all the dates in the app are momentjs dates.

Comienza con la documentación de MatDatepicker y MatMomentDateModule y, después, revisa el issue #7167 relacionado para consultar la discusión existente sobre zonas horarias. Documenta una forma clara y compatible de obtener fechas independientes de la zona horaria o fechas UTC, incluido el comportamiento esperado para los valores de moment; el issue se considera terminado cuando los usuarios pueden seguir las indicaciones sin depender de comentarios dispersos del issue.

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