[docs]: `<rh-avatar>` demos missing css fallbacks

Tracked by #3119

Fallbacks are missing in inline css

- https://github.com/RedHat-UX/red-hat-design-system/blob/c3abf27e811bdb4f0606d860c24cbfe4bad57732/elements/rh-avatar/demo/layouts-in-narrow-containers.html#L54
  ```css
      --rh-color-interactive-primary-hover-on-dark: var(--rh-color-accent);
  ```