Introduce CODEOWNERS for easier reviews

[Mikaela]

As we haven't been too active lately, I don't know who to poke for reviews and I don't know if I come across as bothersome. Thus I would like to have reviewing to be automatic.

This requires that:

• organization members actually join https://github.com/orgs/sailfishos-community/teams/reviewers
• the team is publicly visible
• @olpeh grants the team write access to this repository.

[Mikaela]

I forgot to say that the documentation about this is at https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners