docs(domain): Add the course, lesson, and enrollment lifecycles

[ebouchut]

Move the three Mermaid state diagrams from issue #42 into the
architecture overview: student course progress, course, and lesson.
Annotate which states v1 persists (DRAFT/PUBLISHED/ARCHIVED for courses
and lessons; ENROLLED/IN_PROGRESS/COMPLETED/DROPPED for enrollments) and
which states stay aspirational (prerequisites, hard deletion).

Fixes #42

---

This is part 3 of 5 in a stack made with GitButler:

•  5  feat(markdown): Render lesson Markdown to sanitized, cached HTML #90
•  4  feat(course): Add the course, lesson, and enrollment domain #89
•  3  docs(domain): Add the course, lesson, and enrollment lifecycles #88 (This PR)
•  2  docs: Document Mailpit and the password reset flow #87
•  1  feat: password-reset #86

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (4e0326d) to head (7da7f94).
⚠️ Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##                dev      #88   +/-   ##
=========================================
  Coverage     80.58%   80.58%           
  Complexity       51       51           
=========================================
  Files            16       16           
  Lines           206      206           
  Branches         12       12           
=========================================
  Hits            166      166           
  Misses           28       28           
  Partials         12       12           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.