As per the update in #8916, our core blocks now use Material Design icons. This PR updates our design doc about block design to reflect that.

Old:

Blocks should have an identifying icon, ideally using a single color. Try to avoid using the same icon used by an existing block. Look to Dashicons for style inspiration.

New:

Blocks should have an identifying icon, ideally using a single color. Try to avoid using the same icon used by an existing block. The core block icons are based on Material Design Icons. Look to that icon set, or to Dashicons for style inspiration.