Create the cross-stage feature discovery moment experiment

What does this MR do and why?

Adds a new experiment for testing the effectiveness of a feature discovery moment (FDM) dashboard accessible from the ? menu in the top navbar. This MR creates the absolute smallest MVC possible. Future MRs will add more details & imagery to the Feature Discovery page, working toward the MVE version (visual design).

Screenshots

New menu item	The new FDM dashboard

Screen recordings

• General flow of the experiment (YouTube link)
• Snowplow tracking events (YouTube link)
• Creating new leads records in CustomersDot (private YouTube link)

How to set up and validate locally

1. Enable billing plans & trials: Gitlab::CurrentSettings.update(check_namespace_plan: true)
2. Turn on the experiment: Feature.enable(:cross_stage_fdm)
3. Create a new group
4. Navigate to a group-related page for that group, such as localhost:3000/<group-path>
5. Click on the (?) menu in the top navbar
   • You should see the “Deliver software faster” (later renamed “Discover Premium & Ultimate”) menu item
6. Click on the “Deliver software faster” (later renamed “Discover Premium & Ultimate”) menu item
   • You should be taken to the new dashboard page
7. Try out the “Start a free trial” and “Contact sales” buttons
   • If you have CustomersDot installed & configured to work with your GDK instance, you can even verify the new leads records that are created as you complete the first step of the trial creation process or as you successfully fill out the PQL (hand-rase lead) form
   • If you have Snowplow Micro enabled, you can watch as the various tracking events are received

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348494 (closed)