Re-add Product Analytics feature flag and update docs

What does this MR do and why?

Since Product Analytics (PA) is being deprioritised, we're moving PA back into closed beta behind a default-off feature flag.

Adding the feature flag will make it clear to self-managed instances that we don't currently support it. We can manually enable it for specific projects on .com as needed. There are currently no self-managed or dedicated users of PA.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	Flag disabled	Flag enabled
Listing - PA not set up
Listing - PA set up	(custom dashboards are still there)
Dashboard editor	(!167296 (comment 2131010244))
Dashboards page - Non-PA dashboard
Dashboards page - built in
Dashboards page - Custom	(PA visualizations will error with link to docs)

How to set up and validate locally

1. You need a GitLab Ultimate license.
2. Navigate to Project > Analytics > Analytics dashboards
3. Verify that you can see the VSA dashboard, but can't set up PA.
4. Enable the new feature flag: echo "Feature.enable(:product_analytics_features)" | gdk rails c
5. Follow the guide to enable product analytics on your GDK.
6. Navigate back to Project > Analytics > Analytics dashboards.
7. Click on the alert CTA on the dashboard listing to enable custom dashboards.
8. Verify that you can set up PA, create PA dashboards, and navigate around.

Related to Bring Product Analytics back to a closed beta s... (#495013 - closed)