Add instructions to use mat-icon with alternative Material Icons themes

#### Documentation Feedback

I recently noticed there are currently no instructions in the docs explaining how to use mat-icon with the alternative Material Icons themes (outlined, rounded, two-tone and sharp).

This may be because it is not officially supported, but I happened to find a way to make it work quite simply, so I thought it could be nice to reference this method in the docs.

I had to change the Material Icons font import url to https://fonts.googleapis.com/icon?family=Material+Icons|Material+Icons+Outlined|Material+Icons+Two+Tone|Material+Icons+Round|Material+Icons+Sharp

and then use mat-icon like this: `home`

This has been discussed in a feature request issue: #14853

**Affected documentation page:** https://material.angular.io/components/icon

Start at the affected Material icon documentation page and review the existing mat-icon fontSet guidance. Add instructions for the alternative themes using the import URL and fontSet example from the issue, with any support caveat clarified; the page is done when this usage is documented.

索引モデルが issue の本文から書いたものです。