docs: clarify the contribution process

[petermakowski]

As a developer, I want to have clear guidelines when a component/new styles should be upstreamed to vanilla, and what's the process in detail. What are the criteria for adding such component?

We should clarify the contribution process, including the fact that we want to err on the side of upstreaming new components.

[bartaz]

Yes, I think we can make https://github.com/canonical/vanilla-framework/blob/main/CONTRIBUTING.md a bit more clear in this regard. Currently it focuses on how to run a project, but the section on "reporting bugs" should be extended to clarify how to propose new components or amendments to them.