Update CSM landing page to fix issues and reflect current CSM program

Why is this change being made?

The existing CSM landing page at https://about.gitlab.com/services/customer-success-management/ has some incorrect and out of date content. This MR is a first attempt at improving the content. Additional work will probably be required but I'd like to keep this iterative and address the low hanging fruit first.

Note: the page doesn't reflect the current style of our other pages as well.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this MR
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel