Implement guide for SCU/onyx hybrid apps

[BoppLi]

• write guide how you can integrate core-ui in an onyx blueprint
  • [x] in Blueprint docs
  • [x] explain in the guide that dark-mode is out of scope
• provide stylesheet for mapping core-ui colors with onyx (if possible, detect in CoreUI if onyx is used (e.g. by doing something like :has(.onyx-app) in CSS and apply the stylesheet then so no manually work is needed for the project)
• Follow up: #1682

[BoppLi]

what was used for sitcon (could be used for inspiration, just note that the recommended hybrid approach won't support dark theme):

.dark-theme {
  --brand-primary-0: var(--onyx-color-base-primary-100);
  --brand-primary-1: var(--onyx-color-base-primary-200);
  --brand-primary-2: var(--onyx-color-base-primary-300);
  --brand-primary-3: var(--onyx-color-base-primary-400);
  --brand-primary-4: var(--onyx-color-base-primary-500);
  --brand-primary-5: var(--onyx-color-base-primary-600);
  --brand-primary-6: var(--onyx-color-base-primary-700);
  --brand-primary-7: var(--onyx-color-base-primary-800);

  --brand-exception-1: var(--onyx-color-text-icons-secondary-intense);

  --brand-secondary-1: var(--onyx-color-base-neutral-100);
  --brand-secondary-2: var(--onyx-color-base-neutral-200);
  --brand-secondary-3: var(--onyx-color-base-neutral-300);
  --brand-secondary-4: var(--onyx-color-base-neutral-400);
  --brand-secondary-5: var(--onyx-color-base-neutral-500);
  --brand-secondary-6: var(--onyx-color-base-neutral-600);
  --brand-secondary-7: var(--onyx-color-base-neutral-900);
  --brand-secondary-8: var(--onyx-color-base-neutral-800);

  --dev-secondary-1: var(--onyx-color-base-neutral-900);
  --dev-secondary-2: var(--onyx-color-base-neutral-700);
  --dev-secondary-3: var(--onyx-color-base-neutral-400);
  --dev-secondary-4: var(--onyx-color-base-neutral-800);

  --red-5: var(--onyx-color-base-danger-500);

  overscroll-behavior: none;

  &:is(scu-top-bar) {
    color: var(--onyx-color-text-icons-neutral-intense);
  }
}

[larsrickert]

Live on: https://vue-blueprint.schwarz/migrations/v3.html#coreui-hybrid-mode (only accessible inside Schwarz VPN)

We decided to not map any CoreUI tokens to onyx to:

1. keep it simple
2. do not make "hybrid mode" too pretty so users want to fully migrate to onyx asap
3. not change CoreUI visuals