[REF] website_slides: improve course frontend page

PURPOSE

Improve onboarding and user experience and course frontend

SPECIFICATIONS

Overall improvements and onboarding :
  * new and improved default images for both course types;
  * preview / free preview badge update;
  * copy writing and spacing update;
  * allow to specifically choose a slide to promote. Display promoted slide is
    not optional anymore as configured in backend;
  * display a sample of lessons if no lesson in a course;

LINKS

Task ID-2241513
PR #55698