"Related billing" cross-link card for detail screens.

Threads the billing graph so no detail page is a dead end: a Customer links to its subscriptions/invoices, an Invoice to its customer/subscription/charge, a Charge back to its invoice, and so on. Renders nothing when there are no links.

<RelatedResources.related_resources items={[
  %{icon: :users, label: "Customer", value: "Acme Corp", href: ~p"/customers/123"},
  %{icon: :invoices, label: "Invoices", value: "3 open", href: ~p"/invoices?customer=123"}
]} />