[Sigmon-241] update contributing manual

Context

• https://its.cern.ch/jira/browse/SIGMON-241
• The contributing manual is outdated nad must be updated for the users to contribute in the API.

What was done

• Update the manual so users can contribute to the API
• Create a branch
• Create a merge request
• releases

Review objectives

• identifying potential refactorings / logic common to other projects
• knowledge sharing