Document compensation, benefits, and perks
Created by: nicksnyder
Benefits were copied from https://github.com/sourcegraph/careers and I will delete the information there (and link to here) once this PR is merged.
I know that we will be re-thinking benefits as part of https://github.com/sourcegraph/about/pull/347 but I think it is good to get this merged here first, and then we can propose changes via PRs to the handbook.