Update and expand the code intelligence team roadmap

Created by: creachadair

Our current team roadmap is out-of-date.

In addition, during our recent discussion on planning & organization we identified a need to document a higher level view of our plans. What we call a "Roadmap" today is a list of work items, some of which are concretely tied to product deliverables and some of which are more aspirational.

I wrote https://github.com/sourcegraph/about/pull/784 as a first attempt at documenting some of our higher-level aspirations, but I think we should probably expand on this also.