Restructured README file for Onboarding

[schrpa]

Moved the content of the README.md license section to the about section of the repository:
"License of our Project (mainly AGPL v3)"
This issue was discussed with Stefan and we decided to add the AGPL v3 license to the about section and remove it from the README.md.

Added onboarding.md file to implement onboarding step-by-step guide later. Needed now to enable linking from README file (start contributing link).

Changed README file as discussed in the prototype.

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