Existing Post Types:
A "post type" is just a template living intemplates/html/components/forms/.
Every post picks exactly one of them — it's what the Content Template dropdown
selects when you edit a Series.
Create a new Post Type:
This writes a new template into the forms directory currently in use (<: $forms_dir :>) along with a matching .json sidecar describing
any custom fields, so that the data model knows to keep them when a post is saved.