Add metadata to System hooks page

What does this MR do?

As part of our efforts to ensure most documentation pages have a recommended TW for questions and issues, this MR updates the System hooks doc page to be associated with the TW for the Manage:Compliance group. This recommendation is based on this comment from #217221 (closed):

I have an open issue that's still fairly immature, but the intent is to add additional events to system hooks that our compliance-minded customers care about. Our customers have complex, in-house tooling built to do things like monitor for certain types of events within GitLab and we're hearing that some of these events they'd like to take action on are missing.

This is something we're trying to solve both by adding new audit events we capture, but also triggering hook events for certain events. This will likely be an ongoing effort for groupcompliance.

(#217221 (comment 346272611))

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

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.