Update roadmap
Created by: nicksnyder
This PR improves the roadmap in a few ways (in my opinion):
- Highlight the things we are actively working on
🏃 - Emphasize directionality rather than timeframe.
- Re-organize to reflect current team ownership and add explicit owners (i.e. I think the teams should be responsible for keeping this up to date. I removed some completed tasks.
- Removed/update sections that I found confusing because things that were done were conflated with things that we still need to do (e.g. integrations).
- Condense previous release page (https://github.com/sourcegraph/about/issues/108 would make this page obsolete and we could delete it)