Consolidate personas into one place

Currently, GitLab's persona information is located in multiple places:

• https://design.gitlab.com/research/personas/
• https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/

The goal of this issue is simply to consolidate the information into the GitLab Handbook and link to this content as appropriate from design.gitlab.com. There will be a separate issue to rationalize the existing content.