Combine cherry-pick pages and redirect one

What does this MR do?

As part of #22236 (closed), recently completed by groupcode review, I needed to review our cherry-picking instructions to ensure they were in sync with the change in the feature. I've known for a while that we had two cherry-pick pages: one in the user docs and one in the Git topic docs. While looking, I discovered these things:

• The user docs page (https://docs.gitlab.com/ee/user/project/merge_requests/cherry_pick_changes.html) is in the top 15% of viewed pages.
• The Git topic page was not.
• The Git topic page also had only one real subheading left, and it was about cherry-picking from the CLI.
• That one topic could be easily rolled into the other page.

From a SEO angle, I think it makes sense to keep the page in the doc/user hierarchy, redirect the smaller / less-used doc/topics/git page to it, and reduce the number of pages to maintain.

Related issues

• Closes Docs: revert/cherry-pick squashed MRs with FF s... (#439097 - closed)
• Related to Enable cherry-picking for fast-forward commits,... (#22236 - closed)

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