Docs: Group level webhooks do not have logs

What does this MR do?

Adds a note indicating that Group level webhooks lack the recent deliveries view.

The documentation to troubleshoot webhooks only works for instance and project level webhooks currently, with group level webhooks lacking the same support.

Related issues

• Related to #325642 (closed)
• Customer ticket: https://gitlab.zendesk.com/agent/tickets/221505
• Related to gitlab-docs#1047 (closed) - @aqualls filed this issue after searching for group level webhooks and getting an ugly result
• Related to #335040 (closed) which @aqualls created to log docs-technical-debt issues with the webhooks page that are out of scope for this merge request

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.