I have read the about this function and I see you need to parameter capability
to allows the user can see the menu as an admin
or editor
.
Read more the function to know detail: https://developer.wordpress.org/reference/functions/add_menu_page/
And you can try this the following code.
/**
* Register menu for settings form page.
*/
public function register_menu() {
// Add top level menu page.
add_menu_page(
__('Siteimprove Plugin'),
__('Siteimprove'),
'edit_posts',//change this line 'manage_options' to 'edit_posts'
'siteimprove',
'Siteimprove_Admin_Settings::siteimprove_settings_form'
);
}
I hope it works for you. Good luck!