feature(routing): Use the views system to render the index page

This makes it much easier to override. There is no need to
register a page handler and remember to `return true`. Instead,
just override the `resources/index` view.
docs(routing): Update custom_index to demonstrate latest best practices
feature(routing): Move all core pages to resource views
chore(routing): remove admin and cron from resource views

Giving plugins access to change this core logic seems dangerous
chore(routing): stop passing URL segments to resource views

We use set_input/get_input to pass this data. Also this moves breadcrumb
creation into the views.