Part B: Documentation to implement issue with PR

[hohwille]

Task summary

This is Part B from task #1707.
Create a new documentation file IDEasy-contributing-process.adoc and document in step-by-step instructions how to implement an issue starting from where to find the backlog, how to self-assign yourself to the issue, move it to in progress, sync the main branch of your fork with upstream, create a feature branch, do changes, run mvn clean install to test your changes, commit changes, create PR, link PR with issue, assign yourself to PR, assign reviewer, move to team review.

Additional context

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