docs: add transparent button to examples

[levithomason]

Expected

The Segment should have a background color to appear solid.

Actual

The segment only has a border and shadow 😞

Problem

This was never noticed because the example card includes a background color:

Fix

The docs now include a "Transparent" button on each example. We should use this liberally when designing components.

Toggling the example to transparent mode clearly shows the issue:

[codecov]

Codecov Report

Merging #407 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #407   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          41       41           
  Lines        1341     1341           
  Branches      172      197   +25     
=======================================
  Hits         1230     1230           
  Misses        107      107           
  Partials        4        4

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