Fix corporate marketing URLs/name in marketing handbook page

Why is this change being made?

The redirects from !115480 (merged) are all intact and working. While reviewing the marketing landing page, I found some references to /corporate-marketing/ in the URLs, and also in the name for the comms incident plan (which is owned by Corporate Communications). Design and Swag had URLs to broken handbook pages too.

This MR updates the URLs to avoid redirects, and updates the Incident response plan to Corporate Communications.

The only remaining mention of "Corporate Marketing" on the landing page is the project group in https://gitlab.com/gitlab-com/marketing

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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