Fix marketing homepage to match new organization structure

Why is this change being made?

There were multiple "sources of truth" of the marketing department and sub-department layout:

• The top of the handbook sentence about the departments in marketing
• The "Marketing Team Functional Groups" section
• The "Marketing Handbooks" section UNDER the "functional groups" section

This MR seeks to clear up that confusion by:

• Using the sentence at the top as approved by Ashley in !111742 (merged) as the source of truth for departments and their sub-departments
• Changing that sentence slightly as the department is not called Marketing Strategy and Operations it is called Marketing Strategy and Platforms (of which Marketing Operations is a sub-department) so I changed that in the first sentence
• Update the "Marketing Team Functional Groups" section to serve as "Departments" in line with the correct language per our GitLab organization structure
• Move all handbooks for sub-departments and teams UNDER those departments to the appropriate place in that section.
• Remove "Corporate marketing" as department

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• 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
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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