[DOCS] Show selected-row checkmark

Fixes #593

The underlying styles that we import from '@addepar/style-toolbox/onyx/index'
should be modified so that they don't hardcode an absolute URL for the background-image
checkmark svg, but in lieu of that larger-scope fix, this PR copies the checkmark.svg
asset and adds a singular style override to display it.

tests/dummy/app/styles/app.scss

@@ -1,6 +1,17 @@
 @import './tables';
 @import '@addepar/style-toolbox/onyx/index';
 
+// Override the url for the checkmark that indicates row selection.
+// This overrides a style in style-toolbox that uses the absolute url
+// '/@addepar/style-toolbox/assets/images/checkmark.svg'. When addon docs
+// are deployed, the root URL is '/ember-table/', and thus the checkmark asset
+// is not able to be loaded.
+// So instead we override the style and use a checkmark svg that has been added to this project:
+// See https://github.com/Addepar/ember-table/issues/593
+.ember-table .et-toggle-select input[type='checkbox'] + *:after {
+  background-image: url('images/checkmark.svg');
+}
+
 $colors: (
   navy: #001f3f,
   blue: #0074d9,