restructured and reworked contributing documentation

[hohwille]

This PR improves the documentation for contributing developers

Implemented changes:

• Move all contributing documentation to a contributing sub-folder.
• Rename the documentation files to short and speaking names.
• Added some missing aspects to that documentation.
• Split some documents into smaller pieces.

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Checklist for this PR

Make sure everything is checked before merging this PR. For further info please also see
our DoD.

• When running mvn clean test locally all tests pass and build is successful
• PR title is of the form #«issue-id»: «brief summary» (e.g. #921: fixed setup.bat). If no issue ID exists, title only.
• PR top-level comment summarizes what has been done and contains link to addressed issue(s)
• PR and issue(s) have suitable labels
• Issue is set to In Progress and assigned to you or there is no issue (might happen for very small PRs)
• You followed all coding conventions
• You have added the issue implemented by your PR in CHANGELOG.adoc unless issue is labeled
  with internal

[coveralls]

Pull Request Test Coverage Report for Build 23853367041

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 70.643%

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Totals
Change from base Build 23822201076:	0.009%
Covered Lines:	10886
Relevant Lines:	14791

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💛 - Coveralls