Add Technical onboarding (workflows) doc for Secure

Inspired by @gonzoyumo's doc: https://docs.google.com/document/d/1bTS7UOklztvzpcEEYhNelcKwfUm2qRLvKyPNZNsPUvc/edit# I left the "Meetings", "Features", and "Projects" sections apart. I want to iterate on this, and we don't have a solution right now for them. They contain private docs, so they should not go in the handbook. What do you think @dhavens?

For the projects part, I'd like to point to https://about.gitlab.com/handbook/engineering/projects/ directly, once https://gitlab.com/gitlab-org/gitlab-ee/issues/8489 is done