Update Contributor Guide to provide more guidance for volunteers and expectations

[TimMonko]

📚 New content request

With our renewed orientation towards a sustainable roadmap and additional paid roles on the project, we need to update the Contributor Guide. At our roadmap review the following points came up:

1. Set clear expectations for volunteers that developers in residence (not volunteers) are expected to work on core Roadmap items, so volunteers can continue to contribute as desired.
2. Define means for volunteers to participate in and contribute to core dev/ Roadmap work
3. Provide better guidance on how to get new / previous contributors oriented towards issues they can work on, if they need ideas!

One idea is to create a pinned issue in napari/napari that is intended also to be volunteer-facing.

[melissawm]

If there's a way to label specific issues/PRs with a "roadmap" or "grant" label or something so it's clear it's part of a specific project and a dev in residence is expected to work on it, I think that would be great!