docs: clarify the contribution process

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.

No file, test, or entry point is named in the issue. Start by finding the repository's contribution guidance and component or style documentation; done means the criteria and detailed process for upstreaming are clearly documented, including the preference to err on the side of upstreaming new components.

Written by the indexing model from the issue text.