Add GitLab Docs project management details to Handbook

Why is this change being made?

We now have issue boards for work on planning and developing work in the GitLab Docs project, and need to document who's responsible for the work and some details about the boards.

See review app preview.

Closes gitlab-org/technical-writing#149 (closed) gitlab-org/gitlab-docs#771 (closed)

TODO:

• Link from here https://about.gitlab.com/direction/create/gitlab_docs/ and other TW and Docs related pages.
• Get SSE team review.

Author Checklist

• Correct MR template applied (e.g blog post)
• 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