Sidebar (page list) as Gutenberg block

## Problem

Our default layout currently displays a sidebar. The sidebar renders a page list where the pages are children of the parent:

https://github.com/creativecommons/creativecommons-base/blob/f49ec2ac4412e3c6abc87f78eea44942dcd22fd1/page.php#L18-L36

However, having the sidebar doesn't allow Gutenberg blocks to display full-width.

## Description

Determine how best to handle the sidebar menu, such as

- moving it to a custom Gutenberg widget
- using the [default Gutenberg page list block](https://wordpress.com/support/full-site-editing/fse-blocks/page-list-block/) (which doesn't allow specifying a root page)
- making the default template have no sidebar and creating a "page with sidebar" theme

## Related resources
- Upstream issue https://github.com/WordPress/gutenberg/issues/33883