Request new function add_sites_page(...)

[ignatiusjeroe]

On a multisite in the Network Admin area there is a new top menu called 'Sites'. It would be beneficial to have a function that adds submenu items to it. All other Admin top menu's have one except for this page. A couple of examples:

add_dashboard_page()

and

add_users_page()

.

If coded the function( see below). The only thing left to alter by your team is the code comments. The function should be placed somewhere in wp-admin/includes/plugin.php. Below is the function code:

<?php
/**
 * Adds a submenu page to the Multisite Sites main menu.
 *
 * This function takes a capability which will be used to determine whether
 * or not a page is included in the menu.
 *
 * The function which is hooked in to handle the output of the page must check
 * that the user has the required capability as well.
 *
 * @since 6.5.x
 *
 * @param string   $page_title The text to be displayed in the title tags of the page when the menu is selected.
 * @param string   $menu_title The text to be used for the menu.
 * @param string   $capability The capability required for this menu to be displayed to the user.
 * @param string   $menu_slug  The slug name to refer to this menu by (should be unique for this menu).
 * @param callable $callback   Optional. The function to be called to output the content for this page.
 * @param int      $position   Optional. The position in the menu order this item should appear.
 * @return string|false The resulting page's hook_suffix, or false if the user does not have the capability required.
 */
function add_sites_page( $page_title, $menu_title, $capability, $menu_slug, $callback = '', $position = null ) {
        return add_submenu_page( 'sites.php', $page_title, $menu_title, $capability, $menu_slug, $callback, $position );
}

### Ticket: https://core.trac.wordpress.org/ticket/61141

## Description

• This PR introduces a new function add_sites_page() to the WordPress core, addressing the ticket mentioned above.
• This function allows developers to add submenu items to the 'Sites' menu in the Network Admin area of a multisite installation.
• Currently, similar functionality exists for other top-level admin menus, such as add_dashboard_page() and add_users_page(), but not for the 'Sites' menu.

### Changes Made

• Added the add_sites_page() function.

## Testing Instructions

1. Set up a local WordPress multisite installation.
2. Apply the changes from this PR.
3. Use the add_sites_page() function to add a submenu item under the 'Sites' menu in the Network Admin area.
4. Verify that the new submenu item appears and functions as expected.

### Example Usage

A sample usage of the add_sites_page() function:

`php
add_action( 'network_admin_menu', function() {

add_sites_page(

'Custom Sites Page',
'Custom Sites',
'manage_network',
'custom-sites-page',
'custom_sites_page_callback'

);

});

function custom_sites_page_callback() {

Callback function for custom sites page.

}