Reworking roadmap to have short-term high level vision, linking to google doc for details

Created by: christinaforney

This roadmap is intended to be the higher level short term version of the roadmap to communicate to customers and users what Sourcegraph will be like in the next 3-6 months. There is a linked google doc that has all the implementation details/tasks that used to be outlined on this page. I believe this will lead to more frequent updates and better collaboration on the "next most important things".

At the bottom of that google doc, there is a previous version that has more context setting happening around each feature. If the present tense doesn't feel right, I can revisit.

I removed quite a lot from this doc, and if there is anything missing that I should add back in, let me know!