Add richer empty state for product analytics usage quota

What does this MR do and why?

Improves the product analytics usage quota empty state.

• always show product analytics usage quota tab, whether or not product analytics is enabled (still hdie the tab if development feature flag product_analytics_usage_quota is disabled)
• use gl-empty-state component
  • when product analytics is disabled, include CTA to docs for enabling product analytics
  • when enabled but no onboarded projects, include CTA to docs for onboarding projects

Screenshots or screen recordings

Before	After
	(product analytics not enabled on instance)
	(product analytics enabled, but no onboarded projects within this group)

How to set up and validate locally

1. Requires GitLab Ultimate.
2. Follow these instructions to set up Product Analytics in your GDK.
3. Visit a group -> Settings -> Usage Quota (verify tab does not display without FF)
4. Enable product_analytics_usage_quota feature flag
5. Visit a group -> Settings -> Usage Quota (verify "enable product analytics" empty state displays)
6. Enable product analytics
7. Visit a group -> Settings -> Usage Quota (verify "onboard projects" empty state displays)
8. Onboard a project with product analytics + generate events
9. Visit a group -> Settings -> Usage Quota (verify chart renders as expected)

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 #428708 (closed)