Show tree-based navigation of projects and subgroups on the Explore public groups page

Description

We are already working on showing projects of subgroups on the parent group page, due in 9.5. This issue proposes to extend that to the explore groups tab as the same use case and benefits apply.

Proposal

Extend the design implemented in https://gitlab.com/gitlab-org/gitlab-ce/issues/30343 to show tree-based navigation of projects and subgroups on the Explore public groups page.

Links / references

Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/79189.