docs(plans): Add the course management by role plan

[ebouchut]

What needs to be done, per role, on top of the shipped course domain:
students browse, register for, read, and quit courses; instructors
author courses and lessons, archive them, and handle their students;
admins create instructor accounts, archive accounts, and archive any
course or lesson.

The plan verifies that the data model needs no change: archiving and
dropping are already lifecycle states (PublicationStatus.ARCHIVED,
EnrollmentStatus.DROPPED, users.is_active), and the INSTRUCTOR and
ADMIN roles are already seeded. It prescribes four stacked feature
branches (security foundation, student, instructor, admin) with one
atomic commit per task.

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This is part 1 of 2 in a stack made with GitButler:

•  2  docs(architecture): Record the shipped course domain #94
•  1  docs(plans): Add the course management by role plan #95 (This PR)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.30%. Comparing base (a29a6a2) to head (0531543).

Additional details and impacted files

@@            Coverage Diff            @@
##                dev      #95   +/-   ##
=========================================
  Coverage     81.30%   81.30%           
  Complexity       62       62           
=========================================
  Files            23       23           
  Lines           246      246           
  Branches         13       13           
=========================================
  Hits            200      200           
  Misses           32       32           
  Partials         14       14           

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