Improve JSON handling

Created by: dominikwerder

Description of work

• Replace usage of the rapidjson library by json for modern c++
• Replacements in source code, build setup, conanfiles, documentation

Issue

Closes #122 (closed)

Acceptance Criteria

Everything should work as before.

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Code Review (To be filled in by the reviewer only)

• Is the code of an acceptable quality?
• Do the changes function as described and is it robust?

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Indicate if you think the code should be reviewed in a Thursday code review session.

• Recommend for group code review

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