docs: restructure "Development" section

[lars-reimann]

Summary of Changes

• Group testing related pages together
• Add link to contributing page

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ MARKDOWN	markdown-link-check	8		0	0.31s
✅ REPOSITORY	git_diff	yes		no	0.04s

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3d99f8d) to head (da43afe).
Report is 133 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1033   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           99        99           
  Lines        18954     18954           
  Branches      3900      3900           
=========================================
  Hits         18954     18954           

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[lars-reimann]

🎉 This PR is included in version 0.12.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• npm package (@latest dist-tag)
• v0.12.0
• GitHub release

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