Illustration > Add more grid information

Purpose

In !2636 (merged) the product illustration guidelines were updated. Based on some feedback there could be more helpful guides specifically around the grid.

A link to some resource would be helpful, or at least some examples of illustrations within this grid so it's easier to see what are the focal points and how elements should align to it. https://design.gitlab.com/product-foundations/illustration#focal-point-grid

This grid is specific to illustration, so I wouldn't assume all Product Designers know how to use it if they haven't had experience with graphic design or illustration.