Raise ripple from core to src/lib

[jelbourn]

• Move ripple code from src/lib/core to src/lib directly
• Add tsconfigs
• Add ripple.md overview
• Rework public API for easier-use.
• Generate material-ripple.html for docs output.
• Add live examples
• Add ripple to docs-items on material.angular.io

[devversion]

The ripple directive is still part of core/, but is documented now. It also includes live examples.

We didn't move it into a separate entry-point because the <mat-option> directive within core/ also depends on the ripples.

So, that would mean that we either need to extract MatOption into a separate module, or the @angular/material/core would depend on @angular/material/ripple. This feels a bit odd.