docs-bug(button): Change icon button size missing

### Documentation Feedback

There is only an example to change some colours of a button (https://material.angular.io/components/button/styling) but no example of changing the size.
Especially for icon buttons, different sizes are often needed. Right now we are using something like that:
```
.mdc-icon-button.small {
line-height: 18px;
.mat-icon {
width: 18px;
height: 18px;
font-size: 18px;
}
}
```
which is probably not the preferred way. I also tried it with:
```
.small {
@include mat.icon-button-overrides((
icon-size: 18px
));
}
```
which doesn't work as expected.


### Affected documentation page

https://material.angular.io/components/button/styling

Start with the affected button styling documentation page and compare its existing color-customization example with the reported icon-button Sass snippets. Determine the supported way to customize icon-button size, document it with a working example, and verify that the rendered example matches the intended sizes.

Written by the indexing model from the issue text.