Add link to informal communication page on Communication Handbook

GitLab's Communication Handbook lacked a lack/mention of Informal Communication, a vital part of GitLab culture. This merge request remedies this.

This MR adds a link to the informal communication handbook page. There is currently no link to informal communication anywhere within this main communication page. For this MR, the link has been placed under the sub-header for Internal Communication since that seems most relevant. This also means the link will come before the other mentions of informal communication (mostly in regards to Slack) that occur further down the page so this can help give some context to informal communication at GitLab.

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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