Create a contributor's guide (CONTRIBUTING.md)

[flaviolivolsi]

As mentioned in PR #2072 a contributor's guide would be useful. This issue is to track that effort.

As a new contributor I ran into a few points of uncertainty that a guide could clarify. Having a CONTRIBUTING.md would make it easier for new people to submit high-quality PRs and reduce the review burden on maintainers.

Some potential topics to cover:

• Branching strategy: clarification on the default branch for PRs (currently seems to be dev even though it might be just temporary).
• Migrations: guidelines on creating and testing migrations for existing installations
• Code style: clarification on any linters, formatters or general style guidelines in use if any
• Setting up a dev environment: any specific steps needed to test changes locally

Happy to help contribute to a draft once a direction is decided!

[victoralensai]

This would be greatly appreciated! :D

Maybe also add small descriptions of what is the purpose of some folders (for example i was confused why there is a config/waybar but also a defaults/waybar).

[brunny-eth]

happy to help here as well if you need a pair of hands @flaviolivolsi