Add specificity to type of navigation approval types (add Settings/Top level) and dates

Why is this change being made?

• Adding more specificity to our due date list so that it is clearer where we are in the process.
• Related to this comment from @hsutor
• Added the following definition of what is or is not a navigation approval
  • First, second, and third navigation additions
  • Renaming a navigation item
  • Removing a navigation item
  • Changing the sort order of navigation items
  • Launching an Experiment or Beta feature

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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