docs: Update extending guide

[keydunov]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue Reference this PR resolves

[For example #12]

Description of Changes Made (if issue reference is not provided)

[Description goes here]

[codecov]

Codecov Report

Merging #2160 (3dfeaf7) into master (e66e699) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2160   +/-   ##
=======================================
  Coverage   55.72%   55.72%           
=======================================
  Files         118      118           
  Lines        8609     8609           
  Branches     1885     1885           
=======================================
  Hits         4797     4797           
  Misses       3433     3433           
  Partials      379      379           

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