Move all job descriptions to handbook from careers repo
Created by: christinaforney
- Creates company/careers.md from careers repo README (PR: https://github.com/sourcegraph/careers/pull/126)
- Moves all job descriptions from careers repo into /roles/<job_descriptions>
- Add link "Go back to careers page for all open positions" to bottom of every open req.
- Creates "Roles" page for each
- Creates people-ops/hiring and puts all hiring/interview related pages there
- Moves job description template and reference check questions from interviews repo to handbook.
- Changes about site careers footer link to go to company/careers.md, updates jobs page button to go there also.