Add the www-gitlab-com webhook to documentation

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds gitlab-com/www-gitlab-com project webhook to the documenation

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review