Unify environment variables usage across the project

[aobolensk]

• Use PPC_ prefix for all used environment variables
• Add environment variables documentation page

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.52%. Comparing base (29103af) to head (f5ce70d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #423   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          16       16           
  Lines         259      259           
  Branches       85       85           
=======================================
  Hits          250      250           
  Misses          3        3           
  Partials        6        6           

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