Add references to the triage-ops-playground project

Context

The triage-ops-playground was created, and if useful, it should be mentioned in the docs for others to use.

What does this MR do and why?

Mention the triage-ops-playground for dry-runs.

Expected impact & dry-runs

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

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.