Add 'Asynchronous communication' sub-head and cross-link to Communication Handbook

Add 'Asynchronous communication' sub-head and cross-link to Communication Handbook

Related issues:

1. Async 3.0 survey questions: https://gitlab.com/gitlab-com/chief-of-staff-team/internal-communications/-/issues/58
2. Building upon Async Communications as our Remote Differentiator: https://gitlab.com/gitlab-com/chief-of-staff-team/internal-communications/-/issues/57

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][transparency]
• 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][mr-buddies-slack].
  • 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.