doc: update CONTRIBUTING.md with copyright guidelines

[homksei]

Description

Added a License and Copyright section to the CONTRIBUTING.md file. This section includes guidelines on how to handle copyright notices for new and existing files. It ensures that contributors are aware of the licensing terms and how to properly attribute their contributions.

Note: It might make sense to use a separate COPYRIGHT.md file for this, but I'm not sure as it might be an overhead

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PR should start as a draft, then move to ready for review state after CI is passed and all applicable checkboxes are closed.
This approach ensures that reviewers don't spend extra time asking for regular requirements.

You can remove a checkbox as not applicable only if it doesn't relate to this PR in any way.
For example, PR with docs update doesn't require checkboxes for performance while PR with any change in actual code should have checkboxes and justify how this code change is expected to affect performance (or justification should be self-evident).

Checklist to comply with before moving PR from draft:

PR completeness and readability

• I have reviewed my changes thoroughly before submitting this pull request.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes or created a separate PR with update and provided its number in the description, if necessary.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have added a respective label(s) to PR if I have a permission for that.
• I have resolved any merge conflicts that might occur with the base branch.