Feature/oaf 83 refactor envvar docs

[stevenbal]

Fixes maykinmedia/open-api-framework#83 partially

Related PRs:

• ✨ Add sphinx directives to generate envvar documentation django-common#34
• ♻️ [#83] Move config helper / envvar docs to maykin-common open-api-framework#183

TODO:

• only use docs helpers in ci.txt?

Changes

• Use sphinx directive to generate envvar documentation
• Remove old management command generated docs

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.75%. Comparing base (213a206) to head (714fe9c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   97.75%   97.75%           
=======================================
  Files         230      230           
  Lines       13480    13480           
=======================================
  Hits        13177    13177           
  Misses        303      303           

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