docs: add v3 OpenAPI specifications

[jlabatut]

Update OpenAPI specifications, in order to comply with Polyflix v3 :

• Courses are the only entity now.
• Elements will be stored in sections.
• Elements can be VIDEO, QUIZ and PAGE.
• The Catalog service now stores only elements IDs and type (and their order in the course).
• An element should have a visibility field (either PUBLIC, PROTECTED or PRIVATE).
• A page entity can be created with all associated routes (POST, PUT/{id}, GET/{id}, DELETE/{id}).

[jlabatut]

User stories :

As	I want to be able to
a contributor	create a course containing elements (videos, quizzes, pages) inside sections
a contributor	update a course
a contributor	delete a course
a member / a guest	access a course, browse its content (sections/elements)
an admin	manage the course (update/delete it)