Improve CS role differentiation and Success Tiers documentation across handbook

Summary

Following the CSA handbook page update (now merged), this epic tracks the remaining documentation improvements to ensure all CS role pages, the Success Tiers page, and the Accelerator catalog are connected, consistent, and reflect how GitLab's paid Success Tiers model actually operates.

Background

An analysis of the CS handbook identified that:

• The Success Tiers page is a two-sentence stub despite the full model being public on about.gitlab.com/services/
• No single page compares all customer-facing CS and SA roles
• The CSM and CSE pages do not reference the Success Tiers
• The Accelerator catalog is not in the handbook (only on the marketplace subdomain)
• The CSA job family has an incorrect reporting line

The CSA page has been updated and merged. These issues cover the remaining work.

Child Issues

• Expand Success Tiers handbook page
• Create CS & SA Roles comparison page
• Add Success Tier reference to CSM handbook page
• Add Success Tier reference to CSE handbook page
• Create public Accelerator Catalog handbook page

Sequencing

CSA page MR — merged ✅

Ready now (HIGH):

• Issue 1: Expand Success Tiers page — landing page for all CS links
• Issue 2: CS Roles comparison page — standalone, no dependencies

After Issue 1 merges (MEDIUM):

• Issue 3: CSM tier reference — links to Success Tiers page
• Issue 4: CSE tier reference — links to Success Tiers page

Independent (MEDIUM):

• Issue 5: Accelerator Catalog — standalone, no dependencies