Fix most anchor links in page
What does this MR do?
From this report: https://gitlab.com/gitlab-com/gitlab-docs/-/jobs/156705064, I've fixed broken links for a specific page about merge requests that aren't shared with CE docs.
Related issues
Relates to: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/397.
Author's checklist
Review checklist
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Technical writer's review (required)