Add fulfillment slack communication guidelines

Why is this change being made?

With https://gitlab.com/gitlab-org/fulfillment-meta/-/issues/704, we divided slack communication into multiple channels so as to make it easier to grasp information for the relevant audience. Adding documentation regarding the slack channel, their purpose and the guidelines to ask questions.
Updates fulfillment sub department handbook page - https://about.gitlab.com/handbook/engineering/development/fulfillment/
Updates can be reviewed here - https://fulfillment-slack-communication.about.gitlab-review.app/handbook/engineering/development/fulfillment/index.html#communicating-on-slack

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • 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 affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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