Allows to programmatically modify a content block content.
The event handler should accepts an array of parameters with the following keys:
- url - the page URL.
- content - the page content string.
- path - path to the page file (if file-based templates mode is enabled).
- file_based - boolean, determines whether file-based templates mode is enabled.
- code - the content block code.
- page_id - the page identifier.
The event handler should return an array with at least a single element
content containing the updated content.