Add functionality so admin settings pages can have sub pages.

Currently there in the administration settings tree; categories can have both pages and categories as children. Pages can only have settings as children not other pages.

Adding the ability for pages to have other pages as children will add to the flexibility and usability of administration settings.

At the moment the per provider action settings are in the "AI" admin category as they cannot be attached to the provider they belong to in the admin settings. Trying to use the current options and adding these to categories, degrades the usability and makes things harder for admins.

For example, currently in the AI subsystem has a settings page for managing each provider, with the breadcrumb: AI > Manage settings for AI providers

Then if an admin accesses the settings of the "OpenAI API Provider", the breadcrumb is: AI > OpenAI API Provider, when a better experience would be the breadcrumb: AI > Manage settings for AI providers > Open AI API Provider

Then if an admin accesses the settings of the "Generate text", the breadcrumb is completely absent as this page currently needs to be attached to the root admin node (otherwise it completely messes up the AI category display). A better experience would be the breadcrumb: AI > Manage settings for AI providers > Open AI API Provider > Generate text