Create a schedule for milestone planning

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 adds a schedule of tasks for milestone planning to the engineering manager page.

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

Review App

Review App: https://mnohr-cr-em-milestone-schedule.about.gitlab-review.app/handbook/engineering/development/dev/create/code-review/backend/manager/

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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