Add product analytics monthly group usage chart

What does this MR do and why?

Adds a monthly usage chart for the group to the new product analytics usage quota tab.

The chart shows the total events for the group for the current + previous month.

The query / data fetching introduced in this MR will later be replaced when we swap out the two months worth of usage for a whole years usage in #429312 (closed)

Merge request	status
create app structure	✔
projects usage table	✔
projects usage visualization	✔
total group usage visualization	<-- you are here
add richer empty state	⌛
feature flag rollout	⌛

Screenshots or screen recordings

Before	After
	(loading)
	(network error)
	(new docs section)
(group with more than 50 product analytics projects)	(review suggestion from previous MR - chart now limited if too many projects)

How to set up and validate locally

1. Requires GitLab Ultimate.
2. Follow these instructions to set up Product Analytics in your GDK.
3. Enable product analytics
4. Enable product_analytics_usage_quota feature flag
5. Visit a group -> Settings -> Usage Quota (verify empty state)
6. Onboard a project with product analytics + generate events
7. 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 #424153 (closed)