Create a handbook page for Create:Code Review BE Engineering Manager

Why is this change being made?

The current handbook page for the Create:Code Review backend team has grown to a size that makes it hard to use. I plan to start to break this into smaller pages that are hopefully more direct.

This MR creates a new page for the engineering manager. The intent is to have a place that lists out responsibilities and resources for the EM to be more transparent. I will add more to this page, but for now, I have just taken the existing content from the handbook and moved it over.

See https://gitlab.com/gitlab-com/create-stage/code-review-be/-/issues/132

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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