distribution: add product & personas page

Created by: slimsag

I originally wrote this as onboarding material for Gonza in https://github.com/sourcegraph/about/pull/1104

I think this is useful context for anybody to have about the product aspects of Distribution @ Sourcegraph, and see it being especially useful context for any potential product manager joining to suport the Distribution team.

Stacked on top of https://github.com/sourcegraph/about/pull/1971