Adds job family yml entry for Senior Manager, Engineering.

Why is this change being made?

There's no entry for Senior Manager, Engineering: https://about.gitlab.com/job-families/engineering/development/management/senior-manager/

The compensation calculator won't be available for Senior Manager, Engineering without adding this information. I've marked the MR WIP as I'll need help from Total Rewards to complete it.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • 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][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.