Implement rule enforcement to have an issue for each pull request

[frewilhelm]

Description

In yesterday's retro, we discussed that only pull requests that are linked to a corresponding issue should be merged.

However, it does not always make sense to create an issue for a small PR (e.g. to correct typos). We therefore need an opt-out option so that the rule is not enforced.

TODOs:

• Implement a GHA that checks that an issue belongs to every PR
• Block PR from merging, if they do not pass the check successfully
• Add an exception to the implementation that recognises, for example, from the PR title or a PR label that it is the opt-out option.
• ...

Done Criteria

• ...
• Code has been reviewed by other team members
• Internal technical Documentation created/updated
• New / changed code is documented
• Analysis of existing tests (Unit and Integration)
• Unit Tests created for new code or existing Unit Tests updated
• Integration Test Suite updated (includes deletion of existing unnecessary Integration Test and/or creation of new ones if required)
• Enduser Documentation updated (if applicable)
• Successful demonstration in Review