Add TW team workflow labels to Handbook

Why is this change being made?

As part of providing more transparency to our stakeholders, the Technical Writing team will be using workflow labels to describe the status of in-progress and complete issues/MRs, and needs to document the labels and their use, both for the team and for stakeholders.

Mentioning @gl-docsteam for awareness, and for any comments about the Handbook update, and to ensure that the changes cover what we discussed in gitlab-org/technical-writing#166 (closed)

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
• 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.
  • 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.

For help with failing pipelines reach out in #mr-buddies in Slack