docs: Document what Ghost's nodemailer wrapper is doing.

[markstos]

Module are kept concise and point readers to official Ghost mail config docs for configuration details.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.73%. Comparing base (2f40c2b) to head (89139b9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #209   +/-   ##
=======================================
  Coverage   89.73%   89.73%           
=======================================
  Files         112      112           
  Lines        7586     7586           
  Branches     1081     1081           
=======================================
  Hits         6807     6807           
  Misses        771      771           
  Partials        8        8           

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