Add a few items to review during under-performance

Why is this change being made?

While reading our Underperformance handbook page during the Manager Challenge Pilot Program Training Adding a few items for a Manager to review during under-performance.

Considering our current average PTO in 2020 is still relatively low, this combined with a few other items for manager to review during underperformance, could be helpful in the midst of a Pandemic or any time really.

@gl-peoplepartners @emilyplotkin @cteskey kindly review this MR and feel free to include other things to review that I have not thought of.

cc @gl-peopleops @brufener @brittanyr @Josh_Zimmerman

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.