Customizable value stream analytics needs documentation

We have just completed #35823 (closed) and are preparing the release notes MR (gitlab-com/www-gitlab-com!34499 (merged)). Having a documentation page that clarifies as to what options are available would be very helpful. The issue has screenshots (e.g. https://gitlab.com/gitlab-org/gitlab/uploads/d4280665724e914514e1217b56e80a5a/ca-custom-stages__add--start-event.png) which illustrate some of the event types we support, but it would be helpful to have a complete list and some scenarios to help new users get started.

@ahegyi previously proposed scenarios such as:

• Have an example company (group) that has many projects.
• Show that we have default stages and how they might not be a perfect fit for everyone.
• Setting up customized stages (events).
• Setting up customized stages (labels).