Reservair/includes/RsvAdminMenuDefinition.php

<?php

/**
 * Contains definitions of the admin menus
 */
function rsv_admin_menu_definition() {
    $reservations = new RsvReservationsPage();
    $forms        = new RsvFormsPage();
    $timetable    = new RsvTimetablePage();
    $google_cal   = new RsvGoogleCalendarSettingsPage();

    add_menu_page(
        'Reservations Settings',     // Page title
        'Reservations',              // Menu title
        RsvCapabilities::MANAGE,     // Capability
        'reservations-settings',     // Menu slug
        [$reservations, 'render'],   // Callback
        'dashicons-calendar',        // Icon
        20                           // Position
    );

    add_submenu_page(
        'reservations-settings',
        'Forms',
        'Forms',
        RsvCapabilities::MANAGE,
        'forms-settings',
        [$forms, 'render']
    );

    add_submenu_page(
        'reservations-settings',
        'Timetables',
        'Timetables',
        RsvCapabilities::MANAGE,
        'timetable-settings',
        [$timetable, 'render']
    );

    add_submenu_page(
        'reservations-settings',
        'Google Calendar',
        'Google Calendar',
        RsvCapabilities::MANAGE,
        'rsv-google-calendar',
        [$google_cal, 'render']
    );
}