Add component sections/shortcuts feature

[bglw]

Often page builders have to make a choice between exposing "fine" components to the editor (title, button) or exposing "coarse" components to the editor (landing-page-hero, two-column-product-info).

A useful Bookshop feature would be the ability to build a "Section" to your array structures, which is a pre-configured set of fine components that could then be edited further. In this world the editor would select landing-page-hero, and added to the page would be (for example):

- _bookshop_name: two-columns
  left_column:
    - _bookshop_name: title
      # ... 
    - _bookshop_name: button
      # ...
  right_column:
    - _bookshop_name: image
      # ... 

At which point the editor could go in and add/remove/edit the "fine" components. This saves them having to crate these page sections by hand, but retains the full control if desired.

I think this can be implemented acceptably without CMS work. Bookshop can create an array structure, which outputs a structure matching two-columns in this example. Once added, the editor would not see the landing-page-hero component referenced at all, so perhaps it's more of a shortcut than a section.