Feature/oaf 83 refactor envvar docs

[stevenbal]

Fixes maykinmedia/open-api-framework#83 partially

Related PRs:

• ✨ [#7] Add envvar config helper + sphinx directives to generate 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 97.72%. Comparing base (2b7b4f6) to head (1a1e24e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   97.72%   97.72%           
=======================================
  Files         210      210           
  Lines       11776    11806   +30     
=======================================
+ Hits        11508    11538   +30     
  Misses        268      268           

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