Add roll up direction for code graph

Created by: jyavorska

This is a roll-up page for the Code Graph organization, which is meant to describe the unifying mission, vision, and themes in the product for us. It will be informed by each of the teams creating their individual direction pages from the template at https://about.sourcegraph.com/handbook/product/product_management/direction_template (see https://about.sourcegraph.com/handbook/engineering/code-graph/batch-changes/direction for a completed example).

What's intended here is to make a clear step forward on defining and communicating our direction as we understand it today, but not necessarily get everything right forever on the first iteration. We'll review these every month as a team and make small, meaningful deltas.