Custom post type name vs. page permalink conflict (same slug)

The simplest solution is to name one “docs” and the other “documentation”, or anything similarly different.

The more complex solution is to add a query to pre_get_posts to resolve the sub-pages if they exist.

This snippet, if installed as a plugin for example to wp-content/plugins/documentation-pages/documentation-pages.php, or to your theme’s functions.php, or to a Code Snippet plugin, should do it:

<?php
/**
 * Plugin Name: Documentation Pages Query
 * Description: Allow "documentation" sub-pages to resolve when "documentation" post type also exists.
 * Plugin Author: 𝜑δ•ℂᴹ
 * Author URI: 
 * Plugin URI: 
 * Version: 1
 */

add_action(
	'pre_get_posts',
	function( $query ){
		if ( $query->is_main_query() && ! is_admin() ) {
			$documentation_page_id = get_posts([
				'post_type'      => 'page',
				'posts_per_page' => 1,
				'post_status'    => 'published',
				'fields'         => 'ids',
				'name'           => $query->get('documentation'),
				'post_parent__in'    => get_posts([
					'post_type'      => 'page',
					'posts_per_page' => 1,
					'post_status'    => 'published',
					'fields'         => 'ids',
					'name'           => 'documentation'
				]),
			]);
			
			if ( ! empty( $documentation_page_id ) ) {
				$query->set( 'post_type', 'page' );
			}
		}
	}
);