Draft: Update Operate onboarding template

This change updates a team onboarding template to reflect the combination of 2 teams.

The template also includes several improvements: updated links to reflect new handbook locations and organizational structure, expanded Slack channel recommendations with more specific team channels, and added new onboarding sections focused on Reference Architectures and GitLab Environment Toolkit training.

Additionally, the access request process has been updated to use a new repository location, some outdated resources have been removed, and the overall structure has been reorganized to be more comprehensive for new team members joining the Operate team.

closes https://gitlab.com/gitlab-org/distribution/team-tasks/-/issues/1784