docs: add course roles doc

[MaferMazu]

Description

Adds a doc on roles and permissions for courses.

Resolves #279

How to test

Review the doc: https://docsopenedxorg--293.org.readthedocs.build/projects/openedx-authz/en/293/concepts/core_roles_and_permissions/course_roles.html and compare it with the roles and permissions in the console or in the wiki page.

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[BryanttV]

Thanks @MaferMazu!, I have a few comments

[rodmgwgu]

Looking good!, just please remove the courses.view_global_staff_and_superadmins permission as it's not implemented. I've updated the wiki doc to reflect this. Thanks!

[BryanttV]

I have no additional comments. Thank you very much!

[bmtcril]

LGTM!