Docs: Update handbook with team-specific labels

Why is this change being made?

To better ensure that the TW team uses labels consistently, and to help our stakeholders know more about how we handle the issues and MRs that we work on, we should document in the Handbook those labels that we regularly use to describe the type of documentation work we do, beyond those labels that are already described in https://docs.gitlab.com/ee/development/contributing/issue_workflow.html .

I augmented the section of the page that linked to the Issue workflow doc page, and added the team-specific, scoped docs labels.

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