Remove Corporate Marketing handbook; update all incoming handbook references

Why is this change being made?

In a coffee chat about practical handbook edits, @rclayman @bbula @jmatthewj told me that the corporate marketing handbook provides duplicated and outdated information. I was under the impression that it only collects sub-pages information, which was already moved out in #13991 (closed)

After removing the handbook page, I've reviewed all incoming references to /handbook/marketing/corporate-marketing/ and updated them accordingly. The data/redirects.yml file is left intact to keep the remaining existing redirects for bookmarks and external incoming links.

When applicable, Corporate Communications was added instead of Corporate Marketing. Same goes for Corporate Events and Brand, depending on the context in the handbook.

I used the search result from VS Code as a local GUI this time. This was faster to review and edit, than the CLI I showed in the coffee chat earlier ;-)

Unfortunately, the changes are larger because VS Code automatically cleans trailing whitespaces in my setup.

I've also removed stale images from source/images/handbook/corporate-marketing, and moved the remaining images for social marketing into a new home in source/images/handbook/integrated-marketing/social-marketing, with updating the image embed locations.

There were a few locations in the community relations handbook that link to corporate marketing OKRs, or otherwise outdated content. That has been removed as well.

Will add suggestions for review in the comments, pinging for review.

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, and the content is SAFE
• 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
• 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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