docs: use relative URLs in documentation

Related to https://github.com/angular/material.angular.io/issues/623, the docs all have routes to other components hard-coded to the https://material.angular.io domain, rather than relative to the hosted documentation.

Please update them to use relative routes.

Start by searching the documentation for hard-coded https://material.angular.io links and identify the affected routes. Update those links to relative routes, then verify that navigation still points to the corresponding hosted documentation paths.

Written by the indexing model from the issue text.