Add CI page in the documentation

[aobolensk]

• Briefly describe CI
• Attach CI graph
• Add CI graph automatic generation script to 'scripts' directory

close #190

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.94%. Comparing base (f1d983a) to head (b7aec41).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   93.94%   93.94%           
=======================================
  Files          26       26           
  Lines         479      479           
  Branches      143      143           
=======================================
  Hits          450      450           
  Misses         14       14           
  Partials       15       15           

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