[feature request] add search box in material.angular.io

#### Bug, feature request, or proposal:
Feature Request

#### What is the expected behavior?
Want a search box in the official doc website, so that we can search relative docs/apis

#### What is the current behavior?
Nothing

#### 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?

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

#### Is there anything else we should know?

Start by inspecting the material.angular.io documentation website and its existing search or navigation entry points; the issue names no repository files or tests. Clarify the relative documentation and API search scope, then consider the work complete when a search box is available on the official site and returns the intended docs and API results.

Written by the indexing model from the issue text.