De-clutter 2d_gizmos example

[rparrett]

Objective

Fixes #13395

Solution

A minimal set of changes that should make the example a bit more readable without totally redesigning it.

• Prune the top left instruction text and make its style consistent with other examples.
  (see standardize instructions in examples #8478)
• Remove one of the circles, which was demonstrating "not overriding segments."
• Make the grid a more subtle color
• Make the rectangles larger and remove their rotation, moving the arc to make room

Before / After