Replace Company Announcement doc with whats happening at gitlab channel

Why is this change being made?

We use the #whats-happening-at-gitlab channel in place of the Company Announcements doc.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what