GitLab: Make a GitLab contribution

Contributor Success at GitLab found that first-time contributors had trouble with the long and intimidating process that getting started seemed to entail.

After some discussion with the team to assess the pain points and desired outcomes, I edited the documentation in parts: first to clarify and reorganize steps, highlighting a more consistent path; then to separate the single page into multiple so that a user can follow a more cohesive workflow.