How we can get the code of edited pages through the admin panel?
I assume that you want to track which admin user made changes in the CMS page (and at which date and time).
For this, you can create a module and observe the event cms_page_prepare_save or model_save_before (or maybe model_save_after) to get the information whether the data has changed or not.
In the observer class, you can use:
$object = $observer->getEvent()->getObject();
to get the object of event, and then use:
to get the data before saving.
which is going to be saved.
I hope the above information is sufficient to start.