Update Snowplow Guide page based on initial review
This page describes product documentation work that was identified during a documentation review of documentation pages associated with the Growth:Product Intelligence stage/group.
- Page has known problems, but these observations should be added to: #227295
- Revise image to Mermaid diagram - better accessibility, better editability
- Issue is no longer open - update to https://gitlab.com/groups/gitlab-org/-/roadmap?state=opened&sort=start_date_asc&label_name%5B%5D=group%3A%3Aproduct+intelligence