docs: Document Mailpit and the password reset flow

[ebouchut]

---

This is part 2 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
•  2  docs: Document Mailpit and the password reset flow #87 (This PR)
•  1  feat: password-reset #86

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (a3a0c3e) to head (48c7b31).

Additional details and impacted files

@@            Coverage Diff            @@
##                dev      #87   +/-   ##
=========================================
  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           

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