docs: add notes about development dependencies

[mkniewallner]

Resolves #705.

PR Checklist

• [x] A description of the changes is added to the description of this PR.
• [x] If there is a related issue, make sure it is linked to this PR.
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [x] If you've added or modified a feature, documentation in docs is updated

Description of changes

Add notes about development dependencies, so that users better understand that development dependencies are not considered for DEP002 rule.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.8%. Comparing base (5afea7a) to head (127e248).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #710   +/-   ##
=====================================
  Coverage   92.8%   92.8%           
=====================================
  Files         35      35           
  Lines        920     920           
  Branches     165     165           
=====================================
  Hits         854     854           
  Misses        52      52           
  Partials      14      14           

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[keenanwells-tatari]

🙏 thanks for this change