Growth engineer onboarding improvement

Problem

Growth fullstack onboarding should be reviewed and updated as needed.

Solution(s)

• add experiment guideline doc links
  • covered by https://gitlab.com/gitlab-org/growth/team-tasks/-/blob/master/.gitlab/issue_templates/Fullstack%20Engineering%20Onboarding.md
• add GLEX doc links
  • covered by https://gitlab.com/gitlab-org/growth/team-tasks/-/blob/master/.gitlab/issue_templates/Fullstack%20Engineering%20Onboarding.md
• tee up an initial experiment to execute on with either onboarding buddy or growth team member
• Streamline the steps to set-up GitLab and Snowplow locally
• Add info on UX Reviews