* Move color utility callouts to start of page
Hierarchically/structurally, in the position they are currently at, the two callouts seem like they "belong" just to the "background color" section. Moving them to the start makes it clearer that those two callouts relate to everything in the page (both "Color" and "Background color" classes.
* Change heading level
otherwise the assistive technology callout looks like it's hierarchically under the "Dealing with specificity" heading
- Use a semitransparent gradient from light to dark which works on any background-color
- Store the gradient as a custom property (--bs-gradient)
- Remove `.bg-gradient-*` variants in favour of `.bg-gradient` which works even when `$enable-gradients` are enabled
- Add gradients to navbar, active page links and badges when gradients are enabled
