Telemetry: Document the new 'Topology' usage ping

The following discussion from !32315 (merged) should be addressed:

• @splattael started a discussion: (+2 comments)

  @mkaeppler I've almost missed to ask whether we should add documentation for the new usage ping to https://docs.gitlab.com/ee/development/telemetry/usage_ping.html#usage-statistics-definitions? 🤔

  /cc @alinamihaila

We should do this before the MVC is shipped, which consists at least of:

• !32315 (merged)
• !33191 (merged)
• https://gitlab.com/gitlab-services/version-gitlab-com/-/merge_requests/455

It also came up in an MR review that it is not obvious how to test this. We should document as well how developers can verify changes they make to this logic.