Create InfiniteScroll examples

The GlInfiniteScroll component is almost complete. We just need to create a few more examples to fill in the demo slots in Pajamas website: http://4456656-review-447-create-li0i0e.service-staging.gitlab.org/components/infinite-scroll/#list-count

• add infinite scroll example with “Showing 60 of 100 items” in the list count
• add infinite scroll example with “Showing 60 of 1,000+ items” in the list count
• add infinite scroll example with “Showing 1,200 of 1,000+ items”" in the list count
• add infinite scroll example with “Showing all items” in the list count
• (optional) use the basic example as the component's main story
• enable followsDesignSystem in infinite_scroll.documentation.js
• when the examples are merged and available in https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com, notify UX that they are ready to be integrated in the page

/cc @beckalippert @andyvolpe