Add step to request access to dedicated-ase Slack group

Summary

This MR adds documentation for ASEs managing GitLab Dedicated customers to request access to the @dedicated-ase Slack group.

Changes

  • Added a new section "Request access to the @dedicated-ase Slack group" at the end of the customer onboarding documentation
  • Includes step-by-step instructions for filing an access request using the Slack Request template
  • Specifies the required information (group handle, member email, and action type)
  • Notes that this is only applicable for ASEs managing Dedicated customers

Closes https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/work_items/40451

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