In Drupal 8 -- I want to modify the basic-page content type to be able to support an "accordion field type"
I've seen the list field type - that can have unlimited fields -- but I am looking for a solution that can feature -- header/body - type features.
There is no module to handle this for you, the solution is to create unlimited Entity Reference in your content type, it must have two fields, Title
and Body
as you want, and to convert it to accordions you should customize new entity field theme and implement accordion there.
Another solution is, handle it by https://www.drupal.org/project/views_bootstrap which support accordion or https://www.drupal.org/project/faqfield module :
Features:
Configurable default text formats
Configurable answer widget
Types: Normal textareas, textfields and formatable textareas
Formatable textareas for any Wysiwyg editor
Configurable number of rows for textarea widget
Field formatters
jQuery Accordion UI
Simple themeable text
Definition list (HTML <dl>)
Anchor link list
Accordion display options
Choose first active question
Collapse open questions
Event to open/collapse questions (eg. mouseover, click)