[IMP] website_slides(_*): improve demo data and remove dummy data

PURPOSE

eLearning should have its own application in manager. Indeed this is becoming
a big application and having it embedded inside Website application is not
enough anymore. It should also hold updated and easy-to-use menus,
navigation, actions and views in backend.

SPECIFICATIONS

Remove "Our Documentation" dummy default existing data. It makes no sense
to have void data. Instead just add a small explanation text in eLearning
front-end telling the user to create its first course if he has rights to
do so.

In order to better show the new backend add some demo data for missing
numbers, notably

  * sales: add confirmed sale orders in order to have revenues value;
  * forum: add some posts and answers;
  * quiz: update slide.slide.partner demo data to hold quiz attempts count;

LINKS

Task 1978729
PR #35061

Co-Authored-By: Thibault Delavallée <tde@odoo.com>
Co-Authored-By: Jérémy Hennecart <jeh@odoo.com>