New label-based navigation / groupings #3339
Description
Create & use a component for landing pages to display pages grouped by particular labels, so for example the Transactions reference can show related transactions grouped together in a box or something.
For v1, it can be used just for children of the current page, but we would eventually like to be able to make label landing pages that show every page on the site with a specified label.
It would also be good to be able to iterate over all labels that exist in the child pages so that the entire landing page can be populated automatically.