Add TW Slack channels to handbook

Why is this change being made?

Similar to the work done to rationalize the number of TW team labels we had, this MR adds a canonical list of TW- and documentation-related Slack channels at GitLab.

In some cases, this will involve "merging" some existing channels. In Slack, that means picking one to roll forward with and renaming it. The other channel is then archived.

#tw-* is intended for mainly the TW team. #docs-* is intended for all Documentarian GitLab Team Members (https://www.writethedocs.org/documentarians/).

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